MacOS CI job: do not re-install CMake

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.

Author: tautschnig

.github/workflows/pull-request-checks.yaml

@@ -706,7 +706,7 @@ jobs:
         with:
           submodules: recursive
       - name: Fetch dependencies
-        run: brew install cmake ninja maven flex bison ccache z3
+        run: brew install ninja maven flex bison ccache z3
       - name: Confirm z3 solver is available and log the version installed
         run: z3 --version
       - name: Download cvc5 binary and make sure it can be deployed