iffy and never no longer interleave

[matthiaskrgr]

this was explicitly enabled via rust-lang/homu#180 back then because otherwise rollup=iffy would never be able to land until ALL rollup=nevers were processed

the new queue no longer implements this

[Zalathar]

The current behaviour seems more sensible to me.

PRs with rollup=iffy should be getting a fair chance to run in rollups.