Bots incorrectly assigned "build" label despite no obvious matching rule

For unclear reasons, the bots assigned the `build` label to https://github.com/openjdk/jdk/pull/3172. There are no files in this PR that should match the `build` requirement. Attempts to reproduce this behavior using `git debug mlrules` has failed.

Possibly additional instrumentation is needed to be able to analyze similar failures in the future.