ci: prevent schema from trying to publish existing version (#951)

<!--- Provide a general summary of your changes in the Title above -->

## Description

- Publishing the same version will fail. If we for example retry the
workflow for staging which will use the same tag

<!--- Describe your changes in detail -->

## Related Issue(s)

- #{issue number}

## Verification

- [ ] **Your** code builds clean without any errors or warnings
- [ ] Manual testing done (required)
- [ ] Relevant automated test added (if you find this hard, leave it and
we'll help out)

## Documentation

- [ ] Documentation is updated (either in `docs`-directory, Altinnpedia
or a separate linked PR in
[altinn-studio-docs.](https://github.com/Altinn/altinn-studio-docs), if
applicable)

.github/workflows/action-publish-schema.yml

@@ -16,8 +16,42 @@ permissions:
   id-token: write
 
 jobs:
+  check-published-version:
+    runs-on: ubuntu-latest
+    outputs:
+      version-exists: ${{ steps.check-published-version.outputs.version-exists }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          registry-url: 'https://registry.npmjs.org/'
+
+      - name: Fetch package name
+        id: fetch-package-name
+        run: echo "PACKAGE_NAME=$(jq -r '.name' package.json)" >> $GITHUB_ENV
+        working-directory: ${{ env.WORKING_DIRECTORY }}
+
+      - name: Check if version exists on NPM
+        id: check-published-version
+        run: |
+          if npm view "${{ env.PACKAGE_NAME }}@${{ inputs.version }}" > /dev/null 2>&1; then
+            echo "version-exists=true" >> $GITHUB_OUTPUT
+            echo "Version ${{ inputs.version }} already exists"
+          else
+            echo "version-exists=false" >> $GITHUB_OUTPUT
+            echo "Version ${{ inputs.version }} does not exist"
+          fi
+        env:
+          NODE_AUTH_TOKEN: ${{ secrets.NPM_TOKEN }}
+
   publish-schema-to-npm:
     runs-on: ubuntu-latest
+    needs: check-published-version
+    if: needs.check-published-version.outputs.version-exists == 'false'
     steps:
       - name: Checkout code
         uses: actions/checkout@v4