JMH Plugin : This is a forkArg not a compiler option (#3957)

#3578 set out to allow passing JVM flags (for example
`--add-modules=jdk.incubator.vector`) into the JVM used by the JMH
plugin when trying to benchmark something in mill.

That implementation kind of works, - it uses `javaCoptions` to pass to
the JVM args though, which isn't very idiomatic mill. Runtime arguments
are usually passed as `forkArgs`. This PR seeks to use 'forkArgs'
instead of the 'javaCoptions' of the mill module.

Both are a list of strings in the end, so both work, but I think this
revised version more correctly follows mill nomenclature as forkedArgs
rather than compiler flags.

Apologies for the spamminess.

contrib/jmh/src/mill/contrib/jmh/JmhModule.scala

@@ -72,7 +72,7 @@ trait JmhModule extends JavaModule {
   def generateBenchmarkSources =
     Task {
       val dest = T.ctx().dest
-      val javacOpts = javacOptions().toSeq
+      val forkedArgs = forkArgs().toSeq
       val sourcesDir = dest / "jmh_sources"
       val resourcesDir = dest / "jmh_resources"
 
@@ -90,7 +90,7 @@ trait JmhModule extends JavaModule {
           resourcesDir.toString,
           "default"
         ),
-        jvmArgs = javacOpts
+        jvmArgs = forkedArgs
       )
 
       (sourcesDir, resourcesDir)