ci: save pip cache for multi-python jobs (googleapis#1095)

Author: software-dov

.github/workflows/tests.yaml

@@ -22,6 +22,7 @@ jobs:
         uses: actions/setup-python@v2
         with:
           python-version: 3.8
+          cache: 'pip'
       - name: Install nox.
         run: python -m pip install nox
       - name: Build the documentation.
@@ -38,6 +39,7 @@ jobs:
         uses: actions/setup-python@v2
         with:
           python-version: 3.8
+          cache: 'pip'
       - name: Install nox.
         run: python -m pip install nox
       - name: Check type annotations.
@@ -57,6 +59,7 @@ jobs:
         uses: actions/setup-python@v2
         with:
           python-version: 3.8
+          cache: 'pip'
       - name: Install system dependencies.
         run: |
           sudo apt-get update
@@ -103,6 +106,7 @@ jobs:
         uses: actions/setup-python@v2
         with:
           python-version: 3.8
+          cache: 'pip'
       - name: Copy mtls files
         run: cp tests/cert/mtls.* /tmp/workspace/tests/cert/
       - name: Install system dependencies.
@@ -161,6 +165,7 @@ jobs:
         uses: actions/setup-python@v2
         with:
           python-version: ${{ matrix.python }}
+          cache: 'pip'
       - name: Install system dependencies.
         run: |
             sudo apt-get update
@@ -191,6 +196,7 @@ jobs:
         uses: actions/setup-python@v2
         with:
           python-version: 3.8
+          cache: 'pip'
       - name: Install system dependencies.
         run: |
             sudo apt-get update
@@ -224,6 +230,7 @@ jobs:
         uses: actions/setup-python@v2
         with:
           python-version: 3.8
+          cache: 'pip'
       - name: Install system dependencies.
         run: |
             sudo apt-get update
@@ -254,6 +261,7 @@ jobs:
         uses: actions/setup-python@v2
         with:
           python-version: 3.8
+          cache: 'pip'
       - name: Install system dependencies.
         run: |
             sudo apt-get update
@@ -277,6 +285,7 @@ jobs:
         uses: actions/setup-python@v2
         with:
           python-version: ${{ matrix.python }}
+          cache: 'pip'
       - name: Install pandoc
         run: |
             sudo apt-get update
@@ -301,6 +310,7 @@ jobs:
         uses: actions/setup-python@v2
         with:
           python-version: ${{ matrix.python }}
+          cache: 'pip'
       - name: Install pandoc
         run: |
             sudo apt-get update
@@ -354,6 +364,7 @@ jobs:
         uses: actions/setup-python@v2
         with:
           python-version: 3.8
+          cache: 'pip'
       - name: Install autopep8
         run: |
             python -m pip install autopep8