Fix --watch failing on invalid PathWatchers.Event & skip wonky tests on Mac CI

[Gedochao]

It seems that swoval's PathWatchers.Event sometimes returned a path, for which the RelPath we checked in the event filter didn't have an available last segment (as it was probably empty) and reacted with a PathError. 🤡
Which was then hard to see, as it was hidden behind a timeout in BloopTests.Restart Bloop server while watching.
I can't replicate this locally.
Example failure: https://github.com/VirtusLab/scala-cli/actions/runs/6683093329/job/18159052355#step:5:503

os.PathError$LastOnEmptyPath: empty path has no last segment
	at os.PathError$LastOnEmptyPath$.apply(Path.scala:225)
	at os.BasePathImpl.last$$anonfun$1(Path.scala:207)
	at scala.Option.getOrElse(Option.scala:201)
	at os.BasePathImpl.last(Path.scala:207)
	at os.BasePathImpl.last$(Path.scala:187)
	at os.RelPath.last(Path.scala:256)
	at scala.build.Build$.isScalaFile$1(Build.scala:708)
	at scala.build.Build$.$anonfun$31(Build.scala:710)
	at scala.build.Build$.doWatch$1$$anonfun$1$$anonfun$1(Build.scala:722)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.build.Build$$anon$1.onNext(Build.scala:1180)
	at scala.build.Build$$anon$1.onNext(Build.scala:1179)
	at com.swoval.files.Observers.onNext(Observers.java:31)
	at com.swoval.files.SymlinkFollowingPathWatcher$1.onNext(SymlinkFollowingPathWatcher.java:58)
	at com.swoval.files.SymlinkFollowingPathWatcher$1.onNext(SymlinkFollowingPathWatcher.java:36)
	at com.swoval.files.Observers.onNext(Observers.java:31)
	(...)

This should hopefully address this (or at the very least give us a meaningful exception if it still fails).

Beyond that, RunTestsDefault.watch artifacts & BloopTests.Restart Bloop server while watching will temporarily be skipped for Mac OS only (until we find a way to make those pass reliably on Mac CI, as they've been making intermittent errors a headache)