Fixes <https://bugs.gnu.org/39727>.
Reported by Paul Garlick <pgarlick@tourbillion-technology.com>.
* nix/libstore/gc.cc (LocalStore::removeUnusedLinks) [HAVE_STATX]: Add
'statx_flags' static variables. Clear 'AT_STATX_DONT_SYNC' flag from
'statx_flags' when 'statx' returns EINVAL.