Using a sha1 of the command combined with the build ID is not a
particularly good or unique identifier:
* A build could fail, be restarted, and then succeed -- assuming no
configuration changes, the sha1 hash of the command as well as the build
ID will be the same. This would lead to an overwritten log file.
* Allowing user input to influence filenames is not the best of ideas.
fc3cf4ecb2