Make GHCup isolate installs non-overwriting by default

ghcup's behavior for cabal / hls / stack now in isolated installs is overwriting. This MR changes that to non-overwriting.

if file already exists at the given isolated location, it will error and report so.

lib/GHCup/Utils/File/Common.hs

@@ -104,3 +104,6 @@ findFiles path regex = do
   contents <- listDirectory path
   pure $ filter (match regex) contents
 
+
+checkFileAlreadyExists :: (MonadIO m) => FilePath -> m Bool
+checkFileAlreadyExists fp = liftIO $ doesFileExist fp