Use ccache in codecov-coverage-report GitHub action

[tautschnig]

The GitHub action takes by far the longest, and while the execution time
likely is dominated by test execution we should not unnecessarily waste
time for compiling.

Performance data will be added to the commit message.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #5909 (a0d1047) into develop (e5e4a18) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5909   +/-   ##
========================================
  Coverage    73.55%   73.55%           
========================================
  Files         1430     1430           
  Lines       155222   155222           
========================================
  Hits        114170   114170           
  Misses       41052    41052           

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[NlightNFotis]

Absolutely needed, thanks for the addition of this!