fix(ci): disable sccache when secrets not avail

[Yohe-Am]

• Makes sccache optional so PRs from dependabot and forks can still run the test suite.
• Increases sccache allotment to 50g.

[Yohe-Am]

Hmm, I wonder why the test suite is not triggering on this PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.11%. Comparing base (bbef981) to head (a7a6b01).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #874      +/-   ##
==========================================
+ Coverage   77.09%   77.11%   +0.01%     
==========================================
  Files         149      149              
  Lines       17849    17843       -6     
  Branches     1730     1732       +2     
==========================================
- Hits        13761    13759       -2     
+ Misses       4065     4061       -4     
  Partials       23       23              

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