`trigger_files` removes labels added by user as part of PR creation

[camelid]

I added some labels as part of the PR creation flow, but rustbot removed them when it added T-rustdoc: rust-lang/rust#91876 (comment)

[Mark-Simulacrum]

Distributed systems problem, I suppose. The github side event log shows:

fd73ee70-5c5d-11ec-914a-1521518a34ea: 16:45:30 - camelid, labeled = +C-cleanup, I-compiletime, I-compilemem
fd7e9cd0-5c5d-11ec-8087-4a7cb5275989: 16:45:30 - camelid, labeled = C-cleanup, I-compiletime, +I-compilemem
fd8812b0-5c5d-11ec-9b60-669487d92dd5: 16:45:30 - camelid, labeled = C-cleanup, +I-compiletime, I-compilemem
fe2208c0-5c5d-11ec-859c-7d384597c723: 16:45:31 - rustbot, unlabeled, remove: C-cleanup, left: T-rustdoc
fe2208c0-5c5d-11ec-859c-7d384597c723: 16:45:31 - rustbot, unlabeled, remove: I-compiletime, left: T-rustdoc
fe2b7ea0-5c5d-11ec-9d4a-c8dfb48b4f18: 16:45:31 - rustbot, unlabeled, remove: I-compilemem, left: T-rustdoc
fe3c6e90-5c5d-11ec-8d28-edd2327c1b85: 16:45:31 - rustbot, labeled = T-rustdoc
fd1a0ea0-5c5d-11ec-8965-864abb4f8d0b: 16:45:32 - camelid, opened, labels = []

Triagebot logs (edited to strip details) show (note, UUIDs here are triagebot, not expected to match those generated by GitHub).

23d83c5e-42b2-4073-bef6-a8945105213f: Opened, labels: []
23d83c5e-42b2-4073-bef6-a8945105213f: set_labels rust-lang/rust#91876 to [T-rustdoc]
5f13fdab-1c90-4e77-a2e9-5c697bc040a6: Labeled, +C-cleanup, labels: [C-cleanup, I-compiletime, I-compilemem]
955c9052-63ea-41b1-b988-ebcc8b58b191: Labeled, +I-compilemem, labels: [C-cleanup, I-compiletime, I-compilemem]
df1cb36a-5984-494e-967d-b8333beb1304: Labeled, +I-compiletime, labels: [C-cleanup, I-compiletime, I-compilemem]
a70da24b-df42-4884-94bc-30b16e87f91c: Unlabeled, -C-cleanup, labels: [T-rustdoc]
4dc2f136-5c14-44b2-baef-4245a6cacfca: Unlabeled, -I-compiletime, labels: [T-rustdoc]
ae0627e2-ca61-416a-91c0-f7b4291a2fed: Unlabeled, -I-compilemem, labels: [T-rustdoc]
8f010590-1d05-4634-a111-3b62f88ce127: Labeled, +T-rustdoc, labels: [T-rustdoc]

Both logs are ordered "oldest" to "newest", i.e., last event is latest event.

I think the likely fix here is to migrate to the new(?) API GitHub offers for adding labels, rather than setting them. It seems like we can do so by just changing the put to a post. We'll want to update the code that calls in to delete and add labels to use the "add" and "delete" APIs as well, so that we're not clobbering unrelated state.

Should be a fairly straightforward change.

[Mark-Simulacrum]

OK, filed #1540 which should close this issue. Really a nice cleanup regardless.