Update environment variable for cache reset for consistency (#12455)

Author: akihironitta

.github/workflows/docs-checks.yml

@@ -31,9 +31,9 @@ jobs:
         uses: actions/cache@v2
         with:
           path: ~/.cache/pip
-          key: ${{ runner.os }}-pip-td${TIME_PERIOD}-${{ hashFiles('requirements.txt') }}
+          key: ${{ runner.os }}-pip-td${{ env.TIME_PERIOD }}-${{ hashFiles('requirements.txt') }}
           restore-keys: |
-            ${{ runner.os }}-pip-td${TIME_PERIOD}-
+            ${{ runner.os }}-pip-td${{ env.TIME_PERIOD }}-
 
       - name: Install dependencies
         run: |