Temporarily revert to Tycho 4.0.8 for GitHub Actions builds

GitHub actions builds are currently failing because of invalid fragments
in the classpath when compiling the SWT fragments after updating to
Tycho 4.0.8. This change temporarily reverts the used Tycho verison in
the GitHub Actions workflow to 4.0.8.

.github/workflows/maven.yml

@@ -73,6 +73,7 @@ jobs:
         --fail-at-end
         -DskipNativeTests=false
         -DfailIfNoTests=false
+        -Dtycho.version=4.0.8
         clean install
     - name: Performance tests
       if: contains(github.event.pull_request.labels.*.name, 'performance')