CSmith GitHub action: run apt-get update

[tautschnig]

This is to fix failures to fetch a security update from Ubuntu mirrors. Run apt-get update just like we do in other GitHub actions.

Also fix the Doxygen job configuration, which was also missing the update step before package installation (but seemingly didn't fail thus far).

• 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).
• n/a 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

All modified and coverable lines are covered by tests ✅

Comparison is base (09dca35) 79.09% compared to head (96dc663) 79.09%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8155   +/-   ##
========================================
  Coverage    79.09%   79.09%           
========================================
  Files         1695     1695           
  Lines       196583   196583           
========================================
  Hits        155493   155493           
  Misses       41090    41090           

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