maybePruneOldTraces: bail if modification times change.

We could implement the right thing easily with streams, but we're not
allowed to use streams in this context, and it doesn't seem worth the
bother of doing it any of the hard ways, given that we've only seen this
once in the year the code's been doing this.

Bug: https://issuetracker.google.com/169836837
Test: treehugger
Change-Id: I4ee29916c1b335bebecfe7f72e74ff71e8a24228