tart run: set "prohibited" activation policy when --no-graphics is set

[edigaryev]

This is a missing piece from #935.

I should've been testing the presence of dock icon with tart run --no-graphics instead of tart create --from-ipsw=latest.

Unfortunately the dock icon is still shown for about 10ms even with prohibited, but that's the best we can do, according to https://stackoverflow.com/a/12981068/9316533:

Workaround: Starting as normal app (step 1.) would solve this problem, but will cause another small one, your app icon might appear for a moment in the dock at startup even if you would like to startup without any window shown. (but we can deal with this, not owning the menubar was a bigger problem for us, so we chose this instead)