Add caching for npm modules

Partly for a tiny speedup, partly to avoid stderr output that is
treated as notably error output by github actions.

Author: pimterry

.github/workflows/ci.yml

@@ -18,6 +18,17 @@ jobs:
         with:
           node-version: 14
 
+      - name: Get npm cache directory
+        id: npm-cache
+        run: |
+          echo "::set-output name=dir::$(npm config get cache)"
+      - uses: actions/cache@v2
+        with:
+          path: ${{ steps.npm-cache.outputs.dir }}
+          key: ${{ runner.os }}-node-${{ hashFiles('**/package-lock.json') }}
+          restore-keys: |
+            ${{ runner.os }}-node-
+
       - run: npm ci
 
       - run: npm run build