Use mold for the coverage build.

[joka921]

In the past, we have seen random crashes of the coverage build and suspected OOM problems on GitHub actions.
The reason probably is that the coverage build links all the tests into a single binary, and uses a debug build for exact line information. Both of these factors increase the RAM usage of the linker. To mitigate this problem, we use the mold linker, which typically has a much lower RAM footprint than the default linker.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.11%. Comparing base (1213437) to head (dfcd087).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1823      +/-   ##
==========================================
- Coverage   90.14%   90.11%   -0.03%     
==========================================
  Files         399      399              
  Lines       38213    38220       +7     
  Branches     4281     4283       +2     
==========================================
- Hits        34448    34443       -5     
- Misses       2473     2484      +11     
- Partials     1292     1293       +1     

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