nixpkgs-unstable channel: don't depend on the metrics job

This reverts commit c23e9e12.

At this moment the benchmarking machine ("t2a") is unreachable from
outside due to IPv6 issues.  (the "t4a" and "t4b" builders as well)
But even generally I can't see why this job should block channels,
provided that it won't remain broken over long term.