nix-profile.sh: Don't set NIX_REMOTE on single user installations

Commit 6a214f3e06 reused the NixOS
environment initialisation for nix-profile.sh, but this is
inappropriate on systems that don't have multi-user support enabled.

scripts/nix-profile.sh.in

@@ -43,7 +43,7 @@ fi
 
 # Set up secure multi-user builds: non-root users build through the
 # Nix daemon.
-if test "$USER" != root; then
+if [ "$USER" != root -a -e @localstatedir@/nix/daemon-socket/socket ]; then
     export NIX_REMOTE=daemon
 else
     unset NIX_REMOTE