Remap the exec root out of build script rustflags

[cpcwood]

Fixes issue: #4201

Build scripts that invoke rustc themselves bake the exec root into their output, because cargo_build_script does not include --remap-path-prefix in the CARGO_ENCODED_RUSTFLAGS it passes to them.

rustix's build.rs feature probe pipes source to rustc on stdin, so the working directory is the only path input rustc has, and it folds that into the --emit=metadata output written to $OUT_DIR. That is a declared build-script output, hashed into the cache key of every dependent crate, so the same source built at two different absolute paths produces different cache keys for the whole dependent chain.

construct_arguments() already applies --remap-path-prefix to every rustc invocation the ruleset constructs, "For determinism to help with build distribution and such". Build scripts reach rustc through CARGO_ENCODED_RUSTFLAGS rather than construct_arguments(), so they never inherited it. rustdoc is the only other exception and is documented as one.

Reproduction, with a Bazel-free minimal check and a four-workspace Bazel proof: https://github.com/cpcwood/bazel-rules-rust-rustix-cache-bust-demo

probe output
  unpatched  e27190bc...  01079d73...   differ
  patched    7e887e12...  7e887e12...   identical

Rustc actions re-executed at a second path
  unpatched  3   (rustix, //:mylib, //:myapp)
  patched    0

Verified on aarch64-darwin and ubuntu-latest. Hash values are platform-specific.

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[UebelAndre]

Thanks! I had jut one question 😄

[UebelAndre]

Thank you!