Add i686 and armv7 musl build via container

[jdygert-spok]

This PR adds an i686 and armv7 musl build to CI by using a container, ala #216 (comment).

The ci config is a little jank, but I think it is the best way to do without having to duplicate the steps or move things around too much.

Closes #216.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (855ec0f) 30.78% compared to head (7083d0c) 30.55%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2008      +/-   ##
==========================================
- Coverage   30.78%   30.55%   -0.24%     
==========================================
  Files          51       51              
  Lines       19539    19564      +25     
  Branches     9426     9454      +28     
==========================================
- Hits         6015     5977      -38     
+ Misses       7906     7905       -1     
- Partials     5618     5682      +64     

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[jdygert-spok]

I'm silly, #216 asks for i686, I will add that as well.

[jdygert-spok]

Artifacts seem right

sccache-i686: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), statically linked, stripped
sccache-armv7: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV), statically linked, stripped

[sylvestre]

thanks

[jdygert-spok]

Could we get a release with these new artifacts?

[sylvestre]

sure

[jdygert-spok]

Thanks!

[sylvestre]

done:
https://github.com/mozilla/sccache/releases/tag/v0.7.5