CI: use Python 3.12 on macOS

... so that it works with Boost Python and does not fail with:
```
-- Found Boost: /opt/homebrew/lib/cmake/Boost-1.86.0/BoostConfig.cmake (found version "1.86.0") found components: filesystem program_options regex system json nowide
-- Found Python3: /opt/homebrew/Frameworks/Python.framework/Versions/3.13/bin/python3 (found version "3.13.0") found components: Development Interpreter Development.Module Development.Embed
CMake Warning (dev) at src/CMakeLists.txt:129 (find_package):
  Policy CMP0167 is not set: The FindBoost module is removed.  Run "cmake
  --help-policy CMP0167" for policy details.  Use the cmake_policy command to
  set the policy and suppress this warning.

Call Stack (most recent call first):
  src/CMakeLists.txt:165 (build_pycsdiff)
This warning is for project developers.  Use -Wno-dev to suppress it.

CMake Error at /opt/homebrew/lib/cmake/Boost-1.86.0/BoostConfig.cmake:141 (find_package):
  Found package configuration file:

    /opt/homebrew/lib/cmake/boost_python-1.86.0/boost_python-config.cmake

  but it set boost_python_FOUND to FALSE so package "boost_python" is
  considered to be NOT FOUND.  Reason given by package:

  No suitable build variant has been found.

  The following variants have been tried and rejected:

  * libboost_python312-mt.dylib (3.12, Boost_PYTHON_VERSION=3.13)

  * libboost_python312-mt.a (3.12, Boost_PYTHON_VERSION=3.13)

  * libboost_python312.dylib (3.12, Boost_PYTHON_VERSION=3.13)

  * libboost_python312.a (3.12, Boost_PYTHON_VERSION=3.13)

Call Stack (most recent call first):
  /opt/homebrew/lib/cmake/Boost-1.86.0/BoostConfig.cmake:262 (boost_find_component)
  /opt/homebrew/Cellar/cmake/3.30.5/share/cmake/Modules/FindBoost.cmake:610 (find_package)
  src/CMakeLists.txt:129 (find_package)
  src/CMakeLists.txt:165 (build_pycsdiff)

-- Configuring incomplete, errors occurred!
make[1]: *** [all] Error 1
make: *** [distcheck] Error 2
```

Closes: csutils#214

.github/workflows/macos.yml

@@ -31,4 +31,4 @@ jobs:
           brew install boost boost-python3 help2man
 
       - name: Build and check
-        run: make distcheck
+        run: make distcheck CMAKE='cmake -DPython3_EXECUTABLE=/opt/homebrew/bin/python3.12'