unix dependency prevents use on Windows

[alt-romes]

We depend on unix currently for System.Posix.Signals. A dependency on unix will prevent haskell-debugger from being used on Windows, where unix is not available.

Don't depend unconditionally on unix!!

[alt-romes]

The code which uses System.Posix.Signals is the call to installHandler, which works around https://gitlab.haskell.org/ghc/ghc/-/issues/4162:

  GHC.runGhc (Just libdir) $ do
    -- Workaround #4162
    _ <- liftIO $ installHandler sigINT Default Nothing

@mpickering should this be run conditionally only when Unix is available?

[alt-romes]

Or should the workaround be extended for Windows too?