Also propogate the Java launcher when copying tasks

We removed the config which ran all JavaExec tasks with the given
launcher, so need to override this again.

A little abusrd that this isn't done by Gradle, but there we go.

build.gradle

@@ -417,6 +417,7 @@ def testServer = tasks.register("testServer", JavaExec.class) {
     exec.copyTo(it)
     setClasspath(exec.getClasspath())
     mainClass = exec.mainClass
+    javaLauncher = exec.javaLauncher
     setArgs(exec.getArgs())
 
     // Jacoco and modlauncher don't play well together as the classes loaded in-game don't