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
```
As far as I know it's greedy but must still match the \\.trace a the end so it'll work. I think this is superceded by @bgertzfield's comment below but let me know if you still want an additional file added to the test.