Add Task.Command#makeExclusive

[alexarchambault]

New attempt at #6928

This adds a makeExclusive helper on Task.Command, that can be used to make a copy of the command with exclusive set to true.

This can be helpful if users want to make existing Mill commands exclusive, like

def testForked(args: String*) =
  super.testForked(args*).makeExclusive(true)
def publishLocal(localIvyRepo: String = null, sources: Boolean = true, doc: Boolean = true, transitive: Boolean = false) =
  super.publishLocal(localIvyRepo, sources, doc, transitive).makeExclusive(true)

This can be useful in some contexts, like if the tests do fancy terminal stuff and cannot run in parallel because of that, or when publishing locally many modules at a time while passing --transitive=true (if several command runs try to publish a given transitive dependency at the same time, they sometimes clash, with one writing files that the other doesn't expect to exist already).

[lefou]

def testForked(args: String*) =
  super.testForked(args*).withExclusive(true)
def publishLocal(localIvyRepo: String = null, sources: Boolean = true, doc: Boolean = true, transitive: Boolean = false) =
  super.publishLocal(localIvyRepo, sources, doc, transitive).withExclusive(true)

Is the resulting binary even calling the super task?

I think I'm open to the idea of this PR, but have issue with the name, which claims to be a harmless property changer, while what we get is a potentially new command with an copied implementation. I'd prefer a name that make this more clear, but I currently don't have an idea. Maybe there are some good naming patterns in the copy-on-write domain?

[alexarchambault]

I initially named it makeExclusive in #6928. I don't mind either way.

[alexarchambault]

Just changed it to makeExclusive again