bootstrap-haskell: XDG: set proper GHCUP_DIR & create it

Fixes XDG install mode.

Without XDG - GHCUP_BIN was creating ~/.ghcup/bin recursively before GHCUP_DIR ~/.ghcup was used. XDG setting exposed that case, because now GHCUP_BIN does not include GHCUP_DIR.

Makes sure that the directory exists before putting a file into it.