Always use sccache's own jobserver

[glandium]

and close the pipes to the inherited jobserver.

[sylvestre]

if i understood correctly, it fixed https://bugzilla.mozilla.org/show_bug.cgi?id=1499382

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 0.00%. Comparing base (0cc0c62) to head (c4e91a6).
Report is 122 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #2296       +/-   ##
==========================================
- Coverage   30.91%       0   -30.92%     
==========================================
  Files          53       0       -53     
  Lines       20112       0    -20112     
  Branches     9755       0     -9755     
==========================================
- Hits         6217       0     -6217     
+ Misses       7922       0     -7922     
+ Partials     5973       0     -5973     

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