Flatten path to bioformats_package.jar in bftools.zip (see gh-901)

Since this uses the native zip command, invoking from the
artifact.dir seems to be the most portable way to get the
unprefixed jar into the zip.
Merge pull request #908 from joshmoore/bftools-fix-again

Flatten path to bioformats_package.jar in bftools.zip (see gh-901)