set QEMU_LD_PREFIX in the runners in order to be able to use env -i (#1420)

I'm working on making assert_cmd working with cross.

There is one case a bit more difficult:
when using `clear_env()` on a process, or `env -i`, the binary is not
able to run anymore because the
`QEMU_LD_PREFIX` envvar is not set anymore.

A (relatively) easy fix might be to ensure in `linux-runner` — and in
any other runner — that the
necessary environment is configured before running the executable.
Something as proposed
in this PR for `aarch64-unknown-linux-gnu`.

Would that be an acceptable solution?

assert-rs/assert_cmd#193

Author: Emilgardis

.changes/1420.json

@@ -0,0 +1,4 @@
+{
+  "description": "set the required environment variables in the runner",
+  "type": "fixed"
+}

ci/test.sh

@@ -243,6 +243,38 @@ main() {
     popd
 
     rm -rf "${td}"
+
+    # test running binaries with cleared environment
+    # Command is not implemented for wasm32-unknown-emscripten
+    if (( ${RUN:-0} )) && [[ "${TARGET}" != "wasm32-unknown-emscripten" ]]; then
+        td="$(mkcargotemp -d)"
+        pushd "${td}"
+        cargo init --bin --name foo .
+        mkdir src/bin
+        upper_target=$(echo "${TARGET}" | tr '[:lower:]' '[:upper:]' | tr '-' '_')
+        cat <<EOF > src/bin/launch.rs
+fn main() {
+    let runner = std::env::var("CARGO_TARGET_${upper_target}_RUNNER");
+    let mut command = if let Ok(runner) = runner {
+        runner.split(' ').map(str::to_string).collect()
+    } else {
+        vec![]
+    };
+    let executable = format!("/target/${TARGET}/debug/foo{}", std::env::consts::EXE_SUFFIX);
+    command.push(executable.to_string());
+    let status = dbg!(std::process::Command::new(&command[0])
+        .args(&command[1..])
+        .env_clear()) // drop all environment variables
+    .status()
+    .unwrap();
+    std::process::exit(status.code().unwrap());
+}
+EOF
+        cross_build --target "${TARGET}"
+        cross_run --target "${TARGET}" --bin launch
+        popd
+        rm -rf "${td}"
+    fi
 }
 
 cross_build() {

docker/Dockerfile.aarch64-unknown-linux-gnu

@@ -51,3 +51,5 @@ ENV CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_LINKER="$CROSS_TOOLCHAIN_PREFIX"gcc \
     CROSS_CMAKE_SYSTEM_PROCESSOR=aarch64 \
     CROSS_CMAKE_CRT=gnu \
     CROSS_CMAKE_OBJECT_FLAGS="-ffunction-sections -fdata-sections -fPIC"
+
+RUN sed -e "s#@DEFAULT_QEMU_LD_PREFIX@#$QEMU_LD_PREFIX#g" -i /linux-runner

docker/Dockerfile.aarch64-unknown-linux-gnu.centos

@@ -46,3 +46,5 @@ ENV CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_LINKER="$CROSS_TOOLCHAIN_PREFIX"gcc \
     CROSS_CMAKE_SYSTEM_PROCESSOR=aarch64 \
     CROSS_CMAKE_CRT=gnu \
     CROSS_CMAKE_OBJECT_FLAGS="-ffunction-sections -fdata-sections -fPIC"
+
+RUN sed -e "s#@DEFAULT_QEMU_LD_PREFIX@#$QEMU_LD_PREFIX#g" -i /linux-runner

docker/Dockerfile.aarch64-unknown-linux-musl

@@ -42,3 +42,5 @@ ENV CARGO_TARGET_AARCH64_UNKNOWN_LINUX_MUSL_LINKER="$CROSS_TOOLCHAIN_PREFIX"gcc.
     CROSS_CMAKE_CRT=musl \
     CROSS_CMAKE_OBJECT_FLAGS="-ffunction-sections -fdata-sections -fPIC" \
     CROSS_BUILTINS_PATCHED_MINOR_VERSION=48
+
+RUN sed -e "s#@DEFAULT_QEMU_LD_PREFIX@#$QEMU_LD_PREFIX#g" -i /qemu-runner

docker/Dockerfile.arm-unknown-linux-gnueabi

@@ -45,3 +45,5 @@ ENV CARGO_TARGET_ARM_UNKNOWN_LINUX_GNUEABI_LINKER="$CROSS_TOOLCHAIN_PREFIX"gcc \
     CROSS_CMAKE_SYSTEM_PROCESSOR=arm \
     CROSS_CMAKE_CRT=gnu \
     CROSS_CMAKE_OBJECT_FLAGS="-ffunction-sections -fdata-sections -fPIC -march=armv6 -marm -mfloat-abi=soft"
+
+RUN sed -e "s#@DEFAULT_QEMU_LD_PREFIX@#$QEMU_LD_PREFIX#g" -i /qemu-runner

docker/Dockerfile.arm-unknown-linux-gnueabihf

@@ -46,3 +46,5 @@ ENV CARGO_TARGET_ARM_UNKNOWN_LINUX_GNUEABIHF_LINKER="$CROSS_TOOLCHAIN_PREFIX"gcc
     CROSS_CMAKE_SYSTEM_PROCESSOR=arm \
     CROSS_CMAKE_CRT=gnu \
     CROSS_CMAKE_OBJECT_FLAGS="-ffunction-sections -fdata-sections -fPIC -march=armv6 -marm -mfpu=vfp"
+
+RUN sed -e "s#@DEFAULT_QEMU_LD_PREFIX@#$QEMU_LD_PREFIX#g" -i /qemu-runner

docker/Dockerfile.arm-unknown-linux-musleabi

@@ -44,3 +44,5 @@ ENV CARGO_TARGET_ARM_UNKNOWN_LINUX_MUSLEABI_LINKER="$CROSS_TOOLCHAIN_PREFIX"gcc
     CROSS_CMAKE_SYSTEM_PROCESSOR=arm \
     CROSS_CMAKE_CRT=musl \
     CROSS_CMAKE_OBJECT_FLAGS="-ffunction-sections -fdata-sections -fPIC -march=armv6 -marm -mfloat-abi=soft"
+
+RUN sed -e "s#@DEFAULT_QEMU_LD_PREFIX@#$QEMU_LD_PREFIX#g" -i /qemu-runner

docker/Dockerfile.arm-unknown-linux-musleabihf

@@ -45,3 +45,5 @@ ENV CARGO_TARGET_ARM_UNKNOWN_LINUX_MUSLEABIHF_LINKER="$CROSS_TOOLCHAIN_PREFIX"gc
     CROSS_CMAKE_SYSTEM_PROCESSOR=arm \
     CROSS_CMAKE_CRT=musl \
     CROSS_CMAKE_OBJECT_FLAGS="-ffunction-sections -fdata-sections -fPIC -march=armv6 -marm -mfpu=vfp"
+
+RUN sed -e "s#@DEFAULT_QEMU_LD_PREFIX@#$QEMU_LD_PREFIX#g" -i /qemu-runner

docker/Dockerfile.armv5te-unknown-linux-gnueabi

@@ -46,3 +46,5 @@ ENV CARGO_TARGET_ARMV5TE_UNKNOWN_LINUX_GNUEABI_LINKER="$CROSS_TOOLCHAIN_PREFIX"g
     CROSS_CMAKE_SYSTEM_PROCESSOR=arm \
     CROSS_CMAKE_CRT=gnu \
     CROSS_CMAKE_OBJECT_FLAGS="-ffunction-sections -fdata-sections -fPIC -march=armv5te -marm -mfloat-abi=soft"
+
+RUN sed -e "s#@DEFAULT_QEMU_LD_PREFIX@#$QEMU_LD_PREFIX#g" -i /qemu-runner