MacOS nightly builds: use Python 3.7 in CI

Nightly builds with MacOS started failing again
due to an upgrade of the default virtual environment
that now uses Python 3.8

This makes us hit #14102 and every build fails. This
commit should be reverted along with the fix to #14102.

.github/workflows/macos_python.yml

@@ -25,6 +25,9 @@ jobs:
     runs-on: macos-latest
     steps:
     - uses: actions/checkout@v2
+    - uses: actions/setup-python@v2
+      with:
+        python-version: 3.7
     - name: spack install
       run: |
         . .github/workflows/install_spack.sh
@@ -37,6 +40,9 @@ jobs:
     timeout-minutes: 700
     steps:
     - uses: actions/checkout@v2
+    - uses: actions/setup-python@v2
+      with:
+        python-version: 3.7
     - name: spack install
       run: |
         . .github/workflows/install_spack.sh
@@ -48,6 +54,9 @@ jobs:
     runs-on: macos-latest
     steps:
     - uses: actions/checkout@v2
+    - uses: actions/setup-python@v2
+      with:
+        python-version: 3.7
     - name: spack install
       run: |
         . .github/workflows/install_spack.sh
@@ -60,6 +69,9 @@ jobs:
     runs-on: macos-latest
     steps:
     - uses: actions/checkout@v2
+    - uses: actions/setup-python@v2
+      with:
+        python-version: 3.7
     - name: spack install
       run: |
         . .github/workflows/install_spack.sh