Add bors to project-stable-mir

Author: oli-obk

repos/rust-lang/project-stable-mir.toml

@@ -1,7 +1,7 @@
 org = "rust-lang"
 name = "project-stable-mir"
 description = "Define compiler intermediate representation usable by external tools"
-bots = ["rustbot"]
+bots = ["bors", "rustbot"]
 
 [access.teams]
 project-stable-mir = "write"