Fix mtime for reproducable toolchains

[nigredon1991]

Hello.
We are running sccache-dist servers in docker and mtime in toolchains is changing, because of this new toolchains appear on the servers.
This request fixes this problem.

[sylvestre]

could you please add a test to make sure we don't regress ?

[nigredon1991]

Yes, of course, I will.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.43750% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 71.57%. Comparing base (2458b62) to head (0eb9329).

Files with missing lines	Patch %	Lines
tests/harness/mod.rs	96.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2430      +/-   ##
==========================================
+ Coverage   71.50%   71.57%   +0.07%     
==========================================
  Files          65       65              
  Lines       36302    36365      +63     
==========================================
+ Hits        25958    26029      +71     
+ Misses      10344    10336       -8     

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[nigredon1991]

Added a test, changed the solution to a more correct one during the process

[nigredon1991]

Thanks for the edits. I fixed everything.