nixos: Don't set !allowSubstitutes (#314664)

It is set by `runCommandLocal` and prevents fetching the build output
from `cache.nixos.org` or another trusted substituter.

nixos/modules/services/continuous-integration/buildkite-agents.nix

@@ -9,7 +9,9 @@ let
         ln --symbolic ${pkgs.writeShellApplication { inherit name text; }}/bin/${name} $out/${name}
       '';
     in
-    pkgs.runCommandLocal "buildkite-agent-hooks" { } ''
+    pkgs.runCommand "buildkite-agent-hooks" {
+      preferLocalBuild = true;
+    } ''
       mkdir $out
       ${lib.concatStringsSep "\n" (lib.mapAttrsToList mkHookEntry hooks)}
     '';