Add support for caching target json specs for Rust compilation.

[ColinFinck]

This has been implemented in a backward-compatible way:

• --target arguments specifying target names are still passed as arguments just like before.
• --target arguments specifying a json file path have previously not been cached at all. The json file is now hashed like other external file dependencies.

Fixes #2174

[sylvestre]

Could you please add a test here to make sure we don't regress?
https://github.com/mozilla/sccache/blob/main/.github/workflows/integration-tests.yml

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 68.42105% with 6 lines in your changes missing coverage. Please review.

Project coverage is 40.65%. Comparing base (0cc0c62) to head (2d81c1e).
Report is 81 commits behind head on main.

Files with missing lines	Patch %	Lines
src/compiler/rust.rs	68.42%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2269      +/-   ##
==========================================
+ Coverage   30.91%   40.65%   +9.73%     
==========================================
  Files          53       54       +1     
  Lines       20112    20767     +655     
  Branches     9755     9654     -101     
==========================================
+ Hits         6217     8442    +2225     
- Misses       7922     8154     +232     
+ Partials     5973     4171    -1802     

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[ColinFinck]

Hey @sylvestre, thanks a lot for your quick response! I've checked the integration-tests.yml file you gave me, but it only contains a single test for each platform/compiler, not for individual flags of the Rust compiler. I'm willing to add a test for the newly supported flag, but I need a good example on how sccache currently does per-flag tests.