util/collect_coverage: Allow overriding TARGET_FILE

[akhramov]

Currently, the instrumented binary is sourced from the TARGET_FILE environment variable. For wrappers around rust_test, this variable points to the wrapper rather than the actual binary, which breaks code coverage.

This change:

• Introduces the RULES_RUST_TEST_BINARY_OVERRIDE environment variable, which wrapper authors can use to supply the path to the original test binary.

• Uses the runfiles library to resolve runfiles_dir instead of relying solely on the RUNFILES_DIR variable.

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[UebelAndre]

Thanks! But I'm confused at what this change does. Is RUNFILES_DIR not reliable? And what is the use of the override?

[akhramov]

Thanks for looking. I owe you a repro on bot of these. Will come up with one soon.

In short,

Is RUNFILES_DIR not reliable

in our setup RUNFILES_PATH is not set when we run bazel coverage, will figure out why first.

And what is the use of the override?

Wrapper scripts have TARGET_FILE set to the wrapper, not the wrapee, so the script passes the wrong path to llvm-cov export