Adapt to changes in getrandom 0.3

This uses a build script to enable the `wasm_js` backend when the
target is exactly `wasm32-unknown-unknown` and the `wasm` feature
of `gix-diff` is enabled.

The recently added step in the CI `wasm` job to enable it for
subsequent steps of that job is accordingly removed.

If successful, this creates a situation roughly similar to what we
had before: authors of crates that use `gix-diff` will not have to
set anything, beyond enabling the `wasm` feature. I don't know if
that is ideal or not; see discussion in #2092. However, it seems
like it is at least as good as how `gix-diff` used `getrandom` 0.2.

Author: EliahKagan

.github/workflows/ci.yml

@@ -457,9 +457,6 @@ jobs:
           for feature in progress parallel io-pipe crc32 zlib cache-efficiency-debug; do
             cargo build -p gix-features --features "$feature" --target "$TARGET"
           done
-      - name: enable wasm_js backend
-        if: matrix.target == 'wasm32-unknown-unknown'
-        run: echo RUSTFLAGS='--cfg getrandom_backend="wasm_js"' >> "$GITHUB_ENV"
       - name: crates with 'wasm' feature
         run: |
           set -x

gix-diff/build.rs

@@ -0,0 +1,7 @@
+use std::env::var_os;
+
+fn main() {
+    if var_os("TARGET") == Some("wasm32-unknown-unknown") && var_os("CARGO_FEATURE_WASM") == Some("1") {
+        println!("cargo:rustc-cfg=getrandom_backend=\"wasm_js\"");
+    }
+}