CI: force macOS to use brew's python at all costs

The default actions one is broken in two ways, and additionally
overwrote homebrew's symlinks to begin with.

.github/workflows/macos.yml

@@ -69,9 +69,15 @@ jobs:
     - uses: actions/checkout@v2
     # Avoid picking up an older version of LLVM that does not work.
     - run: brew update
+    # github actions overwrites brew's python. Force it to reassert itself, by running in a separate step.
+    - name: unbreak python in github actions
+      run: |
+        find /usr/local/bin -lname '*/Library/Frameworks/Python.framework/*' -delete
+        sudo rm -rf /Library/Frameworks/Python.framework/
+        brew install --force python3 && brew unlink python3 && brew unlink python3 && brew link --overwrite python3
     # use python3 from homebrew because it is a valid framework, unlike the actions one:
     # https://github.com/actions/setup-python/issues/58
-    - run: brew install pkg-config ninja llvm qt@5 boost ldc hdf5 openmpi lapack scalapack sdl2 python3 boost-python3 gtk-doc
+    - run: brew install pkg-config ninja llvm qt@5 boost ldc hdf5 openmpi lapack scalapack sdl2 boost-python3 gtk-doc
     - run: |
         python3 -m pip install --upgrade setuptools
         python3 -m pip install --upgrade pip