ci: only create a header update PR if tag does not yet exist

.github/workflows/update-header-pr.yml

@@ -46,7 +46,15 @@ jobs:
         git config user.email "noreply@github.com"
         git add Vulkan-Headers vulkan
         rm -rf build
+        git fetch --all --tags
+        if [ -z $(git tag | grep $VK_HEADER_GIT_TAG) ]
+        then
+          echo "Creating commit for new tag: $VK_HEADER_GIT_TAG"
           git commit -m "Update Vulkan-Headers to $VK_HEADER_GIT_TAG" || echo 'No commit necessary!'
+        else
+          echo "Tag already exists: $VK_HEADER_GIT_TAG"
+        fi
+        git stash
         git clean -xf
 
     - name: Create Pull Request