Sync NIX_PROFILES between single-user and multi-user modes

When we are in single user mode, we still want to have access to
profiles. This way things in Nixpkgs that rely on them getting set
accurately are done in both cases. The point where I hit this is with
using aspell which looks in NIX_PROFILES:

https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/libraries/aspell/default.nix

Before this patch, NIX_PROFILES was never set in single user mode!
This corrects that.

scripts/nix-profile.sh.in

@@ -57,7 +57,7 @@ if [ -n "$HOME" ] && [ -n "$USER" ]; then
 
     # Set up environment.
     # This part should be kept in sync with nixpkgs:nixos/modules/programs/environment.nix
-    NIX_PROFILES="@localstatedir@/nix/profiles/default $NIX_USER_PROFILE_DIR"
+    export NIX_PROFILES="@localstatedir@/nix/profiles/default $HOME/.nix-profile"
 
     # Set $NIX_SSL_CERT_FILE so that Nixpkgs applications like curl work.
     if [ -e /etc/ssl/certs/ca-certificates.crt ]; then # NixOS, Ubuntu, Debian, Gentoo, Arch
@@ -79,5 +79,5 @@ if [ -n "$HOME" ] && [ -n "$USER" ]; then
     fi
 
     export PATH="$NIX_LINK/bin:$__savedpath"
-    unset __savedpath NIX_LINK NIX_USER_PROFILE_DIR NIX_PROFILES
+    unset __savedpath NIX_LINK NIX_USER_PROFILE_DIR
 fi