Share cache entries even if -Cprefer-dynamic and -Cextra-filename differ

[shadaj]

Makes two changes that improve the cache hit rate if switching between compilation with -Cprefer-dynamic and not. First, because sccache only caches rlib / staticlib crates, -Cprefer-dynamic has no effect and can be ignored (it is often set via global RUSTFLAGS).

Similarly, the Cargo-generated fingerprint is easily invalidated by custom RUSTFLAGS, but since sccache has fine-grained knowledge of the rustc invocation it should ignore this fingerprint. We do this by ignoring this flag in the cache key, and also stripping the -Cextra-filename (which Cargo uses to append the fingerprint to outputs) from the keys of the output files.

Most of this behavior is (to my understanding) covered by the existing tests, but I adjusted a hashing test to ensure these two flags are correctly ignored.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.11765% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.19%. Comparing base (d9d2eb6) to head (ebe19a3).

Files with missing lines	Patch %	Lines
src/compiler/rust.rs	94.11%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2469      +/-   ##
==========================================
+ Coverage   71.18%   71.19%   +0.01%     
==========================================
  Files          64       64              
  Lines       35588    35612      +24     
==========================================
+ Hits        25333    25355      +22     
- Misses      10255    10257       +2     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 11 untouched benchmarks

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_{Comparing shadaj:cache-prefer-dynamic-extra-filename (ebe19a3) with main (d9d2eb6)}