nixos-prepare-root: force symlink at /run

Otherwise a reinstall will fail.

Changed files
+1 -1
nixos
modules
installer
+1 -1
nixos/modules/installer/tools/nixos-prepare-root.sh
···
mkdir -m 0755 -p $mountPoint/var
mkdir -m 0700 -p $mountPoint/root
-
ln -s /run $mountPoint/var/run
+
ln -sf /run $mountPoint/var/run
# Create the necessary Nix directories on the target device
mkdir -m 0755 -p \