Also test info.xml validity

Signed-off-by: Raimund Schlüßler <raimund.schluessler@mailbox.org>

.github/workflows/lint.yml

@@ -18,6 +18,13 @@ jobs:
         coverage: none
     - name: Lint
       run: composer run lint
+    - name: Download schema
+      run: wget https://apps.nextcloud.com/schema/apps/info.xsd
+    - name: Lint info.xml
+      uses: ChristophWurst/xmllint-action@v1
+      with:
+        xml-file: ./appinfo/info.xml
+        xml-schema-file: ./info.xsd
 
   node-linters:
     runs-on: ubuntu-latest