ci: use actionlint on GHA workflows

- https://github.com/rhysd/actionlint is cool

.github/workflows/cd.yml

@@ -15,7 +15,7 @@ jobs:
 
     - name: Extract dyndnsd version from tag name
       run: |
-        echo "DYNDNSD_VERSION=${GITHUB_REF#refs/*/v}" >> $GITHUB_ENV
+        echo "DYNDNSD_VERSION=${GITHUB_REF#refs/*/v}" >> "$GITHUB_ENV"
 
     - name: Wait for dyndnsd ${{ env.DYNDNSD_VERSION }} gem to be available
       run: |
@@ -23,6 +23,7 @@ jobs:
         for retry in $(seq 1 5); do
           echo "Checking if dyndnsd $DYNDNSD_VERSION gem is retrievable from rubygems.org (try #$retry)..."
           sudo gem install dyndnsd -v "$DYNDNSD_VERSION"
+          # shellcheck disable=SC2181
           if [ $? -eq 0 ]; then
             exit 0
           fi

.github/workflows/ci.yml

@@ -33,3 +33,13 @@ jobs:
     - name: Lint and Test
       run: |
         bundle exec rake ci
+
+  actionlint:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Check workflow files
+      run: |
+        echo "::add-matcher::.github/actionlint-matcher.json"
+        bash <(curl https://raw.githubusercontent.com/rhysd/actionlint/main/scripts/download-actionlint.bash)
+        ./actionlint

.github/workflows/vulnscan.yml

@@ -16,9 +16,9 @@ jobs:
     steps:
     - name: Install Trivy
       run: |
-        mkdir -p $GITHUB_WORKSPACE/bin
+        mkdir -p "$GITHUB_WORKSPACE/bin"
         echo "$GITHUB_WORKSPACE/bin" >> "$GITHUB_PATH"
-        curl -sfL https://raw.githubusercontent.com/aquasecurity/trivy/master/contrib/install.sh | sh -s -- -b $GITHUB_WORKSPACE/bin
+        curl -sfL https://raw.githubusercontent.com/aquasecurity/trivy/master/contrib/install.sh | sh -s -- -b "$GITHUB_WORKSPACE/bin"
     - name: Download Trivy DB
       run: |
         trivy image --download-db-only