nix-daemon: source nix-profile-daemon.sh only once

Cherry-picked-from: https://github.com/NixOS/nix/pull/12805
Co-authored-by: Qyriad <qyriad@qyriad.me>
Fixes: #1134
Change-Id: I0c575f6b974e06b5064564bb084829fa6a6a6964

scripts/nix-profile-daemon.fish.in

@@ -15,7 +15,7 @@ if test -n "$__ETC_PROFILE_NIX_SOURCED"
   exit
 end
 
-set __ETC_PROFILE_NIX_SOURCED 1
+set --export __ETC_PROFILE_NIX_SOURCED 1
 
 set --export NIX_PROFILES "@localstatedir@/nix/profiles/default $HOME/.nix-profile"
 

scripts/nix-profile-daemon.sh.in

@@ -1,6 +1,6 @@
 # Only execute this file once per shell.
 if [ -n "${__ETC_PROFILE_NIX_SOURCED:-}" ]; then return; fi
-__ETC_PROFILE_NIX_SOURCED=1
+export __ETC_PROFILE_NIX_SOURCED=1
 
 NIX_LINK=$HOME/.nix-profile
 if [ -n "${XDG_STATE_HOME-}" ]; then