fzf: fix fish integration

Since fzf 0.43.0, the fzf_key_bindings function is only defined when
fish is running interactively, see [1].
This caused errors when entering non-interactive fish shells since we
called fzf_key_bindings during startup.

[1]: 7e89458a3b

modules/programs/fzf.nix

@@ -190,7 +190,7 @@ in {
       fi
     '');
 
-    programs.fish.shellInit = mkIf cfg.enableFishIntegration ''
+    programs.fish.interactiveShellInit = mkIf cfg.enableFishIntegration ''
       source ${cfg.package}/share/fzf/key-bindings.fish && fzf_key_bindings
     '';
   };