Added env var to disable multi arch caching

[coretechsec]

Added an environment variable SCCACHE_CACHE_MULTIARCH to enable caching of multiarch compilations.

This should help in cases such as that raised in #847 (comment)

[codecov-commenter]

Codecov Report

Patch coverage: 62.50% and project coverage change: -0.06 ⚠️

Comparison is base (b2faf16) 29.42% compared to head (495c057) 29.36%.

❗ Current head 495c057 differs from pull request most recent head d38b796. Consider uploading reports for the commit d38b796 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1775      +/-   ##
==========================================
- Coverage   29.42%   29.36%   -0.06%     
==========================================
  Files          49       49              
  Lines       17626    17638      +12     
  Branches     8511     8517       +6     
==========================================
- Hits         5187     5180       -7     
  Misses       7339     7339              
- Partials     5100     5119      +19     

Impacted Files	Coverage Δ
src/compiler/gcc.rs	54.72% <62.50%> (-1.31%)	⬇️

... and 11 files with indirect coverage changes

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[Xuanwo]

Thanks!