Disable Fastor in macOS CI job

XCode refuses to compile algebra-plugins with the Fastor plugin turned
on, and this is causing the CI to fail. This commit disables Fastor in
the macOS CI job to ensure that it runs through.

.github/workflows/builds.yml

@@ -41,6 +41,7 @@ jobs:
     # Run the CMake configuration.
     - name: Configure
       run: cmake --preset default-x86-64
+                 -DALGEBRA_PLUGINS_INCLUDE_FASTOR=${{ (matrix.PLATFORM.OS == 'macos-latest') && 'OFF' || 'ON' }}
                  -S ${{ github.workspace }} -B build
                  -G "${{ matrix.PLATFORM.GENERATOR }}"
     # Perform the build.