ci: really, really correct usage for coverity

Author: ethomson

.github/workflows/nightly.yml

@@ -245,7 +245,7 @@ jobs:
       with:
         fetch-depth: 0
     - name: Download container
-      run: ci/getcontainer.sh xenial
+      run: ${{ github.workspace }}/ci/getcontainer.sh" xenial
       env:
         DOCKER_REGISTRY: ${{ env.docker-registry }}
         GITHUB_TOKEN: ${{ secrets.github_token }}