MacOS CI job: do not re-install CMake

[tautschnig]

This causes brew to complain about conflicting taps due to pinning that was done (by GitHub) to alleviate a possible build failure by CMake 4.x. See actions/runner-images#12912, and should be fixed (by GitHub) already, but apparently that's not uniformly the case. The rollout of CMake 4.x is also blocked by those issues (cf. actions/runner-images#12934).

See
https://github.com/diffblue/cbmc/actions/runs/17673397336/job/50229845951?pr=8646 for an example of a failing run on our end.

• 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.
✅ Project coverage is 80.39%. Comparing base (15b0003) to head (cc46051).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8711   +/-   ##
========================================
  Coverage    80.39%   80.39%           
========================================
  Files         1688     1688           
  Lines       207411   207411           
  Branches        73       73           
========================================
  Hits        166757   166757           
  Misses       40654    40654           

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