Set MAVEN_HOME on CI

Author: jjohannes

.github/workflows/ci-build.yml

@@ -4,6 +4,8 @@ on:
     branches: [ main ]
   pull_request:
     branches: [ main ]
+env:
+  MAVEN_HOME: "/usr/share/apache-maven-3.9.9"
 jobs:
   gradle-build:
     runs-on: ubuntu-latest