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 not sure how Java handles this by default, but normally `.+` would match greedily, so any file starting with `build.` would be matched.

I think you might need `build\\..+?\\.trace`.

Just to be sure can you add a test to check it won't match a file called `build.log`?