github actions: use canonical way to get ImageOS

.github/workflows/build.yml

@@ -132,10 +132,12 @@ jobs:
       CPPCHECK_VER: 2.8
       CPPCHECK_REPO: https://github.com/danmar/cppcheck.git
     steps:
-      # This is currently the only way to get a version into
-      # the cache tag name - see https://github.com/actions/cache/issues/543
-      - run: |
-          echo "OS_VERSION=`lsb_release -sr`" >> $GITHUB_ENV
+      # Set steps.os.outputs.image to the specific OS (e.g. 'ubuntu20')
+      # see https://github.com/actions/cache/issues/543
+      - name: Get operating system name and version.
+        id: os
+        run: echo "::set-output name=image::$ImageOS"
+        shell: bash
       - uses: actions/checkout@v2
       - name: Cache cppcheck
         uses: actions/cache@v2
@@ -143,7 +145,7 @@ jobs:
           cache-name: cache-cppcheck
         with:
           path: ~/cppcheck.local
-          key: ${{ runner.os }}-${{ env.OS_VERSION }}-build-${{ env.cache-name }}-${{ env.CPPCHECK_VER }}
+          key: ${{ steps.os.outputs.image }}-build-${{ env.cache-name }}-${{ env.CPPCHECK_VER }}
       - run: sudo scripts/install_cppcheck_dependencies_with_apt.sh
       - run: ./bootstrap
       - run: scripts/install_cppcheck.sh $CPPCHECK_REPO $CPPCHECK_VER