Prevent multiple executions of nix-profile.sh (see #12805)

Add a check to ensure the script executes only once per shell.

This makes bash behave the same way as fish, which checks `__ETC_PROFILE_NIX_SOURCED` both for nix.fish and nix-daemon.fish

Author: juhp

scripts/nix-profile.sh.in

@@ -1,4 +1,8 @@
+# Only execute this file once per shell.
 # This file is tested by tests/installer/default.nix.
+if [ -n "${__ETC_PROFILE_NIX_SOURCED:-}" ]; then return; fi
+export __ETC_PROFILE_NIX_SOURCED=1
+
 if [ -n "${HOME-}" ] && [ -n "${USER-}" ]; then
 
     # Set up the per-user profile.