[github-actions] fix brew install issue on macOS (#6990)

When brew install ninja, python 3.9 is being installed as a
dependency. However, the installation failed because there is a
conflict on binary /usr/local/bin/2to3.

.github/workflows/build.yml

@@ -270,6 +270,7 @@ jobs:
         submodules: true
     - name: Bootstrap
       run: |
+        rm -f '/usr/local/bin/2to3'
         brew update
         brew install automake m4 ninja
         [ ${{ matrix.CC }} != clang ] || brew install llvm

.github/workflows/posix.yml

@@ -230,6 +230,7 @@ jobs:
     - uses: actions/checkout@v2
     - name: Bootstrap
       run: |
+        rm -f '/usr/local/bin/2to3'
         brew update
         brew install ninja socat
     - name: Build