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.
Edited  by Anton Latukha