Add file locking to microfactory

To prevent against multiple instances attempting to rebuild the same
executable, add a file lock to Build().

The trace file may not be consistent in this case, but everything that
reads it should be able to handle a corrupted trace file.

Test: `while true; do <microfactory-enabled executable>; done` in
parallel, and modify the code to microfactory (echo "//"
>>microfactory.go)
Change-Id: I30cb22bb9c790c57c507354127d4a5f82526a489