If the first artifact is specified in a build config with the following:

• Of the form: "some/source/file": ""
• The source file is a single file (not a directory or a pattern that matches multiple files)

Then the first file will be written to <build-dir>/artifacts as a filename rather than a directory.
This makes successive builds fail since the script expects <build-dir>/artifacts to be a directory,
not a file.

Depends on D7353

./build-configurations.py build-source-control-tools
./build-configurations.py build-source-control-tools

Pre-patch, the above fails. Post-patch, it passes. You can also run the first line pre-patch
and the second post-patch and it should also pass.