JMH Plugin : This is a forkArg not a compiler option

[Quafadas]

#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.

[lihaoyi]

@Quafadas could you flesh out the PR description a bit with the summary of the problem and the current status

[Quafadas]

@lihaoyi done.