@rustbot ready and @rustbot author erroring out in a few cases

[JohnCSimon]

on this PR: rust-lang/rust#95696 (comment)

Tried using @rustbot ready shortcut to switch labels and got an error

Error: The "Ready" shortcut only works on pull requests.

I don't know what is different about this pull request that produces such an error.

Zulip:
https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/.40rustbot.20author.20not.20working

[joboet]

Another example: rust-lang/rust#101442 (comment)