ci: do not let bors run if the awaiting-CI label is present (#6068)

We already did this in mathlib3.

This forward-ports leanprover-community/mathlib#9478, which was created a few days after this file was ported in #52.

bors.toml

@@ -1,7 +1,7 @@
 status = ["Build", "Lint style", "Check all files imported"]
 use_squash_merge = true
 timeout_sec = 28800
-block_labels = ["not-ready-to-merge", "WIP", "blocked-by-other-PR", "merge-conflict"]
+block_labels = ["not-ready-to-merge", "WIP", "blocked-by-other-PR", "merge-conflict", "awaiting-CI"]
 delete_merged_branches = true
 update_base_for_deletes = true
 cut_body_after = "---"