fix(nix/buildkite): Do not set 'branches' attribute in step groups

This is no longer accepted by the Buildkite API and causes build
failures.

Functionality is unchanged since we also set the property on the step
itself.

Change-Id: Ib0e0908e4093ca4522711170a7179ce4bacafdc0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5324
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: ezemtsov <eugene.zemtsov@gmail.com>
Tested-by: BuildkiteCI

nix/buildkite/default.nix

@@ -276,7 +276,7 @@ rec {
   # Create a gated step in a step group, independent from any other
   # steps.
   mkGatedStep = { step, label, parent, prompt }: {
-    inherit (step) branches depends_on;
+    inherit (step) depends_on;
     group = label;
     skip = parent.skip or false;