[CI] Sandbox error when running shards testuite on GitHub Actions

I finally managed to get some more useful error messages, for the failure at

BinaryBuilderBase.jl/test/runners.jl

Line 446 in 8d1eca1

@test run(ur, `/bin/bash -c "$(test_script)"`, iobuff; tee_stream=devnull)
rustc -o /tmp/testsuite/x86_64-linux-gnu/rust/hello_world/hello_world hello_world.rs
  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:
  Cross-device link (os error 18)
make[1]: *** [Makefile:7: /tmp/testsuite/x86_64-linux-gnu/rust/cargo_build/cargo_build] Error 101
make: *** [Makefile:48: install-project-rust-cargo_build] Error 2
Sounds like a problem in the sandbox code, which perhaps only arises on the CI setup.

Originally posted by @giordano in #346 (comment)

Note: this happens only with the non-squashfs images with UserNS runners (not Docker).