chore(ci): relink python3 to python3.9 on Homebrew

Implements a work-around for the compatibility issue between mozjs and
Homebrew's Python 3.10 formula.

Author: yvt

.github/workflows/main.yml

@@ -104,6 +104,15 @@ jobs:
       - uses: actions/checkout@v2
         with:
           fetch-depth: 2
+      # TODO: Remove this step when the compatibility issue between mozjs and
+      #       Homebrew's Python 3.10 formula (servo/rust-mozjs#559) is fixed
+      - name: Select Python 3.9
+        run: |
+          brew install python@3.9
+          cd $(dirname $(which python3.9))
+          rm -f python3 pip3
+          ln -s python3.9 python3
+          ln -s pip3.9 pip3
       - name: Bootstrap
         run: |
           python3 -m pip install --upgrade pip virtualenv

.github/workflows/nightly.yml

@@ -39,6 +39,15 @@ jobs:
       - uses: actions/checkout@v2
         with:
           fetch-depth: 2
+      # TODO: Remove this step when the compatibility issue between mozjs and
+      #       Homebrew's Python 3.10 formula (servo/rust-mozjs#559) is fixed
+      - name: Select Python 3.9
+        run: |
+          brew install python@3.9
+          cd $(dirname $(which python3.9))
+          rm -f python3 pip3
+          ln -s python3.9 python3
+          ln -s pip3.9 pip3
       - name: Bootstrap
         run: |
           python3 -m pip install --upgrade pip virtualenv