Cache npm cache between JS build runs (#2710)

.github/workflows/build.yml

@@ -14,6 +14,16 @@ jobs:
       - name: Check out code
         uses: actions/checkout@v2
 
+      - name: Restore npm cache
+        uses: actions/cache@v2
+        with:
+          path: ~/.npm
+          key: ${{ runner.os }}-node-${{ hashFiles('js/package-lock.json') }}
+          restore-keys: |
+            ${{ runner.os }}-node-
+
+      # Our action will install npm, cd into `./js`, run `npm run build`,
+      # then commit and upload any changes
       - name: Build production JS
         uses: flarum/action-build@master
         env: