Update cache keys in the build workflow

Now that we only have one pair of `pipenv` configuration with lockfile
and use the lockfile in the `test` job, then it makes sense to hash the
lockfile and use it as part of the cache key for that job.

.github/workflows/build.yml

@@ -195,7 +195,8 @@ jobs:
             ${{ env.PIP_CACHE_DIR }}
           key: "${{ env.BASE_CACHE_KEY }}\
             ${{ hashFiles('**/requirements-test.txt') }}-\
-            ${{ hashFiles('**/requirements.txt') }}"
+            ${{ hashFiles('**/requirements.txt') }}-\
+            ${{ hashFiles('Pipfile.lock') }}"
           restore-keys: |
             ${{ env.BASE_CACHE_KEY }}
       - name: Install core dependencies