gummiboot-builder.py: run nix with no build-users-group

During install, the bootloader script gets run inside a chroot after the
/etc/group bind-mount is unmounted. Since we're not doing any building,
this should be safe, but really nix should just not care if the group
does not exist when no build is needed.

Fixes #5494

(cherry picked from commit 9245516b46a64316f1d7b0622ff12519a0316044)
Signed-off-by: Domen Kožar <domen@dev.si>