nixos/programs/fish: set absolute path in babelfishTranslate

Babelfish translates `source script.sh` to `argv0 < script.sh | source`,
which is impure, and breaks when babelfish isn't in `systemPackages`.

To fix this, exec babelfish with argv0 set to babelfish from the final
system.

nixos/modules/programs/fish.nix

@@ -48,7 +48,7 @@ let
     path: name:
     pkgs.runCommandLocal "${name}.fish" {
       nativeBuildInputs = [ pkgs.babelfish ];
-    } "babelfish < ${path} > $out;";
+    } "exec -a ${lib.getExe pkgs.babelfish} babelfish < ${path} > $out;";
 in
 {
   options = {