Rust toolchain tests failing with unpriviliged runner and unpacked compiler shards

[giordano]

With

BINARYBUILDER_RUNNER=unprivileged
BINARYBUILDER_USE_SQUASHFS=false

Rust toolchain tests are failing on CI with

  Installing cargo_build v0.1.0 (/usr/share/testsuite/rust/cargo_build)
error: failed to compile `cargo_build v0.1.0 (/usr/share/testsuite/rust/cargo_build)`, intermediate artifacts can be found at `/usr/share/testsuite/rust/cargo_build/target`.
To reuse those artifacts with a future compilation, set the environment variable `CARGO_TARGET_DIR` to that path.

Caused by:
  failed to create directory `/usr/share/testsuite/rust/cargo_build/target`

Caused by:
  Cross-device link (os error 18)
make[1]: *** [Makefile:7: /tmp/testsuite/x86_64-linux-gnu/rust/cargo_build/cargo_build] Error 101

This looks like rust-lang/rustup#2949. This is also similar to #347, which had been marked as solved at some point.

[giordano]

Presumably, setting CARGO_TARGET_DIR to an actually writable directory, as suggested by the error message, could help

CARGO_TARGET_DIR — Location of where to place all generated artifacts, relative to the current working directory. See build.target-dir to set via config.

Edit: this is actually the directory where the files are generated, which we typically expect to the be a subdir of the workspace dir. Other toolchain tests are able to write to that dir, so that shouldn't be the issue per se, I think. Setting CARGO_TARGET_DIR could help for this specific issue in the tests, but in general it'd be confusing for other builds.