Run diff workflow on PRs

[Antikyth]

I imagine this is not possible, given this comment: https://github.com/QuiltMC/quilt-mappings/blob/23w35a/.github/actions/diff-uploader/index.js#L94-L101 But I think it is good to make an issue for this anyway, as if that is true, then that TODO can be removed and replaced by a brief comment explaining why it's not possible.

[Antikyth]

If not, perhaps adding another way for diffs to be viewed would be possible. Like, I know the update base workflow works on PRs.

[OroArmor]

I believe it's a permissions issue that prevents the comment on the pr. I think i said something somewhere, I wonder if I can find it.

[Antikyth]

@rustbot is used in the Rust repo to reply to PRs automatically, and @bors is used in response to command comments to do certain things. Just for reference.

[Antikyth]

@rustbot is used in the Rust repo to reply to PRs automatically, and @bors is used in response to command comments to do certain things. Just for reference.

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