Fix cpp-linter tests to avoid .git dependency

[tautschnig]

Makes sure tests also pass when working from an archive rather than a git clone.

• 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

Patch coverage has no change and project coverage change: +0.47 🎉

Comparison is base (6a689f3) 78.05% compared to head (1aab3ba) 78.53%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7799      +/-   ##
===========================================
+ Coverage    78.05%   78.53%   +0.47%     
===========================================
  Files         1697     1697              
  Lines       193627   193627              
===========================================
+ Hits        151144   152064     +920     
+ Misses       42483    41563     -920     

see 53 files with indirect coverage changes

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