Gradle ITs: fix passing system properties

Currently passes something like `-D=maven.repo.local=/home...`, the `=` is wrong here.

integration-tests/gradle/src/test/java/io/quarkus/gradle/QuarkusGradleWrapperTestBase.java

@@ -104,7 +104,7 @@ private List<String> getSytemProperties() {
     }
 
     private static String toPropertyArg(String name, String value) {
-        return new StringBuilder().append("-D=").append(name).append("=").append(value).toString();
+        return new StringBuilder().append("-D").append(name).append("=").append(value).toString();
     }
 
     private void printCommandOutput(File projectDir, List<String> command, BuildResult commandResult, int exitCode) {