make nixos-install get along with root on tmpfs

[oluceps]

nixpkgs/nixos/modules/installer/tools/nixos-install.sh

Line 138 in 7f1c8f1

tmpdir="$(mktemp -d -p "$mountPoint")"

This will create dir tmp.RANDOM under /mnt (which is the default mount point), since it contains random str, it could not be predicted and mounted. root on tmpfs users may mount it on none or tmpfs (as the default behavior disko does), and may cause some building phases ran out the memory.

Maybe adding a nixos-install cmd line arg to customize the tmpdir.