Meta: GitHub tag action: pass GitLab URL as an env variable

GitHub action variables are not available in called shell scripts. They
have to be explicitly passed from the YAML file. See:
https://github.com/orgs/community/discussions/27027

.github/scripts/tag.sh

@@ -14,5 +14,5 @@ else
     git tag -a ${version} -m "Release ${version}"
     git push origin ${version}
     echo "Pushed new tag:"
-    echo "${{github.server_url}}/${{github.repository}}/tag/${version}"
+    echo "${REPO_URL}/releases/tag/${version}"
 fi

.github/workflows/tag.yaml

@@ -18,3 +18,5 @@ jobs:
 
       - name: Create Tag
         run: .github/scripts/tag.sh
+        env:
+          REPO_URL: ${{github.server_url}}/${{github.repository}}