[actions] add step security runner

.github/workflows/shellcheck.yml

@@ -26,9 +26,18 @@ jobs:
             file: nvm-exec # only runs in bash
 
     steps:
+      - name: Harden Runner
+        uses: step-security/harden-runner@v1
+        with:
+          allowed-endpoints:
+            ghcr.io:443
+            github.com:443
+            pkg-containers.githubusercontent.com:443
       - uses: actions/checkout@v2
       - name: Install shellcheck
         run: brew install shellcheck
+        env:
+          HOMEBREW_NO_ANALYTICS: 1
       - run: "shellcheck --version"
       - name: Run shellcheck on ${{ matrix.file }}
         run: shellcheck -s ${{ matrix.shell }} ${{ matrix.file }}
@@ -39,4 +48,8 @@ jobs:
       needs: [shellcheck_matrix]
       runs-on: ubuntu-latest
       steps:
+        - name: Harden Runner
+          uses: step-security/harden-runner@v1
+          with:
+            egress-policy: block
         - run: 'echo tests completed'