When a PR gets "r-" during the auto build, bors thinks it's a try build

[RalfJung]

This happened in rust-lang/miri#3392:

• oli did "r+"
• bors started the auto build
• I did "r-"
• the auto build finished
• bors thought it was a try build

Somehow bors doesn't seem to keep track of whether the PR is currently try-building or auto-building?
I think this may be the dual issue to #47, where a try build result is interpreted as an auto build result.