Old trace files not being deleted because the filename format has changed

The regex for matching trace files for deletion from buck-out/log/traces was not updated when the filename format changed.

Old format example:
```
build.1009.trace
```
New format example:
```
build.9e4e8dda-fab4-6f83-308d-803a3a1850dc.trace
```
I'm going to close this since we all seem to think that @bgertzfield has fixed this.  If you want to add a test to make sure we don't regress it, we'll happily take it!