chore(nixbuild): add sshKey field and finalize fish init

- Added `sshKey` field for all builder configurations in `nixbuild.nix` to standardize access keys.
- Replaced `programs.fish.shellInit` with `environment.shellInit` and introduced `programs.fish.loginShellInit` for improved Nix path initialization.

systems/nixbuild.nix

@@ -11,6 +11,7 @@ with lib.metacfg;
       maxJobs = 4;
       speedFactor = 3;
       sshUser = "harald";
+      sshKey = "/etc/ssh/nix-builder-key";
     }
     {
       hostName = "rialo";
@@ -18,6 +19,7 @@ with lib.metacfg;
       maxJobs = 4;
       speedFactor = 3;
       sshUser = "harald";
+      sshKey = "/etc/ssh/nix-builder-key";
     }
     {
       hostName = "sgx";
@@ -25,24 +27,31 @@ with lib.metacfg;
       maxJobs = 4;
       speedFactor = 1;
       sshUser = "harald";
+      sshKey = "/etc/ssh/nix-builder-key";
     }
   ];
 
   nix.settings = {
+    /*
       substituters = [
         "https://cache.nixos.org"
         "ssh-ng://harald@m4?ssh-key=/etc/ssh/nix-builder-key"
         "ssh-ng://harald@rialo?ssh-key=/etc/ssh/nix-builder-key"
         "ssh-ng://harald@sgx?ssh-key=/etc/ssh/nix-builder-key"
       ];
-
+    */
     builders-use-substitutes = true;
   };
 
   # Ensure Nix is in PATH for SSH sessions
-  programs.fish.shellInit = ''
+  environment.shellInit = ''
     if [ -e /nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh ]; then
       . /nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh
     fi
   '';
+
+  programs.fish.loginShellInit = ''
+    set -gx PATH /nix/var/nix/profiles/default/bin $PATH
+    set -gx PATH /run/current-system/sw/bin $PATH
+  '';
 }