Spellcheck is PR CI only

[jieyouxu]

To prevent possibility of merging changes with typo, and then all subsequent PRs will have red spellcheck PR CI.

See #t-infra > Spellcheck workflow now fails on all PRs (tree bad?)

[Noratrieb]

It should be part of proper CI in general, it is just a completely random workflow with no regards for existing CI conventions

[jieyouxu]

Oh hang on, I think it was setup like "it can fail but won't block other CI jobs fron proceeding". I'll revisit the MCP.

[RalfJung]

Right now, every PR gets flagged as introducing a typo, even though the typo was introduced by a different PR... so that's no good.

Cc @klensy

[klensy]

Oh, sorry, but #144166 with total no pr checks looks weird.

[RalfJung]

Yeah but that happens every now and then when Github has a minor or major outage, whatever system we build must be resilient against such events.
Only the bors checks can actually be relied upon.