chore(nixbuild): update fish path initialization key

- Replaced `programs.fish.loginShellInit` with `programs.fish.shellInit` for aligning key usage.
- Ensures proper Nix path initialization across fish shell sessions.

systems/nixbuild.nix

@@ -42,7 +42,7 @@
   };
 
   # Ensure Nix is in PATH for SSH sessions
-  programs.fish.loginShellInit = ''
+  programs.fish.shellInit = ''
     set -gx PATH /nix/var/nix/profiles/default/bin $PATH
     set -gx PATH /run/current-system/sw/bin $PATH
   '';