test: add xmllint step before build-test

.github/workflows/linux-eic-shell.yml

@@ -3,8 +3,18 @@ name: linux-eic-shell
 on: [push, pull_request]
 
 jobs:
+  xmllint:
+    runs-on: ubuntu-latest
+    stpes:
+    - uses: actions/checkout@v2
+    - run: |
+        sudo apt-get update
+        sudo apt-get install -y libxml2-utils
+        xmllint *.xml compact/*.xml > /dev/null
+
   build-test:
     runs-on: ubuntu-latest
+    needs: xmllint
     steps:
     - uses: actions/checkout@v2
     - uses: cvmfs-contrib/github-action-cvmfs@v2