update workflow

.github/workflows/bashtest.yml

@@ -5,17 +5,34 @@ on: [push, pull_request]
 jobs:
   lint:
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        os: [ubuntu-latest, ubuntu-20.04, ubuntu-18.04]
 
     steps:
     - name: Checkout repository
       uses: actions/checkout@v2
 
+    - name: Cache ShellCheck
+      id: cache-shellcheck
+      uses: actions/cache@v2
+      with:
+        path: /usr/local/bin/shellcheck
+        key: ${{ runner.os }}-shellcheck
+        restore-keys: |
+          ${{ runner.os }}-shellcheck
+
     - name: Install ShellCheck
+      if: steps.cache-shellcheck.outputs.cache-hit != 'true'
       run: sudo apt-get install -y shellcheck
 
     - name: Lint Bash scripts
       run: |
+        echo "Running ShellCheck..."
         find . -name "*.sh" -print0 | xargs -0 shellcheck -f gcc | tee shellcheck.log
         if grep -q "error:" shellcheck.log; then
+          echo "Linting errors found!"
           exit 1
+        else
+          echo "No linting errors found."
         fi