giomodule: Ignore GIO_MODULE_DIR when running as setuid

Even if the modules in the given directory never get chosen to be used,
loading arbitrary code from a user-provided directory is not safe when
running as setuid, as the process’ environment comes from an untrusted
source.

Also ignore `GIO_EXTRA_MODULES`.

Spotted by Simon McVittie.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #2168