feat(ci): enable pip caching in CI (#388)

Changes made by this PR can be summarized as follows:

- Use an updated version of `actions/setup-python` so as to allow the caching of `pip`.

Author: SauravMaheshkar

.github/workflows/build.yml

@@ -8,9 +8,11 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python
-      uses: actions/setup-python@v2
+      uses: actions/setup-python@v4
       with:
         python-version: '3.8.x'
+        cache: 'pip'
+        cache-dependency-path: setup.py
     - name: Install dependencies
       run: |
         pip install tf-nightly mock pytest