Reduce emscripten cache size by removing everything but llvm, clang and build folders

[mcbarton]

Description

Please include a summary of changes, motivation and context for this PR.

Our cache for our emscripten llvm builds is quite large. This is the first in a series of 2 PRs to reduce its impact on our total cache. This should provide a small speed up in our emscripten builds, since the emscripten llvm uploads and downloads should be quicker.

Fixes # (issue)

Type of change

Please tick all options which are relevant.

• Bug fix
• New feature
• Requires documentation updates

Testing

Please describe the test(s) that you added and ran to verify your changes.

Checklist

• I have read the contribution guide recently

[vgvassilev]

LGTM!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.04%. Comparing base (8e064dc) to head (e2ea792).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #460   +/-   ##
=======================================
  Coverage   71.04%   71.04%           
=======================================
  Files           9        9           
  Lines        3550     3550           
=======================================
  Hits         2522     2522           
  Misses       1028     1028