Move checkstyle to slightly later compile phase

By: md_5 <git@md-5.net>

paper-server/pom.xml

@@ -434,7 +434,7 @@
                         <version>3.1.2</version>
                         <executions>
                             <execution>
-                                <phase>process-classes</phase>
+                                <phase>test-compile</phase>
                                 <goals>
                                     <goal>check</goal>
                                 </goals>