[lfmerge] Run git clean again after preparing source
[lfmerge] Run git clean again after preparing source

Running PrepareSource restores packages which in turn creates
obj subdirectories in the project directories. We don't want these
in the source package, so we run git clean again.

Change-Id: I2d2a08a1f4e2805ab6fe0f7f7a3c156995cd6776

sillsdev/ci-builder-scripts,sillsdev/ci-builder-scripts,sillsdev/ci-builder-scripts