Make the `preparer` details work correctly

I managed to miss that the preparer code was not filling in the string, which
is not in any way fatal, but also doesn't help get the data I wanted embedded
in the final ISO.

This fixes that by using sensible bash syntax to build the variable.

Signed-off-by: Daniel Pittman <daniel@rimspace.net>
Merge pull request #41 from daniel-pittman/feature/master/better-iso-file-metadata

Make the `preparer` details work correctly