nixos-enter: Ensures presence of full /sys tree.

This partially reverts a change from e88f289
which removed the `mount --rbind /sys`.

While true that the activation scripts will mount `sysfs` at `/sys`,
none of the mountpoints lower in the `/sys` tree are handled by the
activation script, which includes `efivarfs`.

This fixes #38477 since it ensures the presence of `efivarfs` in the
`/sys` tree, which is why the systemd-boot installation failed.

nixos/modules/installer/tools/nixos-enter.sh

@@ -51,8 +51,9 @@ if [[ ! -e $mountPoint/etc/NIXOS ]]; then
     exit 126
 fi
 
-mkdir -m 0755 -p "$mountPoint/dev"
+mkdir -m 0755 -p "$mountPoint/dev" "$mountPoint/sys"
 mount --rbind /dev "$mountPoint/dev"
+mount --rbind /sys "$mountPoint/sys"
 
 # Run the activation script. Set $LOCALE_ARCHIVE to supress some Perl locale warnings.
 LOCALE_ARCHIVE=$system/sw/lib/locale/locale-archive chroot "$mountPoint" "$system/activate" >&2 || true