Fix valgrind workflow in CI

[Anders429]

Fixes #65.

Seems valgrind being installed from source is more trouble than it's worth. Since I was installing from source to attempt to resolve an issue with running trybuild tests anyway, which we ended up skipping in the end, this reverts back to installing with apt-get.

[codecov-commenter]

Codecov Report

Merging #68 (a06a904) into master (80f7eca) will not change coverage.
The diff coverage is n/a.

❗ Current head a06a904 differs from pull request most recent head d6011bc. Consider uploading reports for the commit d6011bc to get more accurate results

@@           Coverage Diff           @@
##           master      #68   +/-   ##
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  Coverage   21.68%   21.68%           
=======================================
  Files          55       55           
  Lines        4003     4003           
=======================================
  Hits          868      868           
  Misses       3135     3135           

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