Initial caching work.

.github/workflows/build.yml

@@ -9,6 +9,21 @@ jobs:
       # you must check out the repository
       - name: Checkout
         uses: actions/checkout@v2
+      - name: Cache west modules
+        uses: actions/cache@v2
+        env:
+          cache-name: cache-zephyr-modules
+        with:
+          path:
+            - modules/
+            - tools/
+            - zephyr/
+            - bootloader/
+          key: ${{ runner.os }}-build-${{ env.cache-name }}-${{ hashFiles('app/west.yml') }}
+          restore-keys: |
+            ${{ runner.os }}-build-${{ env.cache-name }}-
+            ${{ runner.os }}-build-
+            ${{ runner.os }}-
       - name: West Init
         uses: ./.github/actions/zephyr-west # Uses an action in the root directory
         id: west-init