Clone gradlex-build-conventions on CI

Author: jjohannes

.github/workflows/build.yaml

@@ -11,6 +11,12 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v5
+      - name: git clone gradlex-build-conventions
+        uses: actions/checkout@v5
+        with:
+          repository: gradlex-org/plugin-publish-conventions
+          path: build/plugin-publish-conventions
+          ref: new-convention-plugins
       - uses: actions/setup-java@v5
         with:
           distribution: temurin