release/pkg-stage: Put each package on a separate line

The list of packages included in dvd1.iso is maintained in pkg-stage.sh
itself, and sees regular additions and deletions. Rearrange the
beginning and end of the variable so that each package is alone on a
line, to make it easier to track changes.

Reviewed by: cperciva
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50221