nix-profile.fish: set --local NIX_LINK

Using `set --local` is better than using `set`/`set --erase`.  `--local`
will preserve any existing `NIX_LINK` value.  And the local variable is
automatically removed for any execution path.

Author: ilya-bobyr

scripts/nix-profile-daemon.fish.in

@@ -24,7 +24,7 @@ end
 
 # Set up the per-user profile.
 
-set NIX_LINK $HOME/.nix-profile
+set --local NIX_LINK $HOME/.nix-profile
 
 # Set up environment.
 # This part should be kept in sync with nixpkgs:nixos/modules/programs/environment.nix
@@ -64,7 +64,6 @@ end
 
 add_path "@localstatedir@/nix/profiles/default/bin"
 add_path "$NIX_LINK/bin"
-set --erase NIX_LINK
 
 # Cleanup
 

scripts/nix-profile.fish.in

@@ -24,7 +24,7 @@ end
 
 # Set up the per-user profile.
 
-set NIX_LINK $HOME/.nix-profile
+set --local NIX_LINK $HOME/.nix-profile
 
 # Set up environment.
 # This part should be kept in sync with nixpkgs:nixos/modules/programs/environment.nix
@@ -63,7 +63,6 @@ if set --query MANPATH
 end
 
 add_path "$NIX_LINK/bin"
-set --erase NIX_LINK
 
 # Cleanup