Improve toolchain cache reliability

[ahartmetz]

These changes do not seem to fix, at least not fully, a problem with broken toolchains that I've observed, but are probably improvements anyway.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.62%. Comparing base (b799af2) to head (a9a728a).

Files with missing lines	Patch %	Lines
src/bin/sccache-dist/build.rs	0.00%	19 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2748      +/-   ##
==========================================
- Coverage   74.63%   74.62%   -0.02%     
==========================================
  Files          70       70              
  Lines       39898    39905       +7     
==========================================
  Hits        29778    29778              
- Misses      10120    10127       +7     

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[sylvestre]

it would be nice to have a test to cover this if this isn't too complex

[ahartmetz]

it would be nice to have a test to cover this if this isn't too complex

It's probably doable, and maybe writing the tests will help me come up with more ideas on how this stuff can fail.