release: amd64: Fix ISO/USB hybrid image

Recent mkimg changes forces to have partitions given in explicit order.
This is so we can have the first partition starting at a specific offset
and the next ones starting after without having to specify an offset.
Switch the partition in the mkisoimage.sh script so the first one created
is the isoboot one.

PR: 254490
Reported by: Michael Dexter <editor@callfortesting.org
Tested by: Vincent Milum Jr <freebsd@darkain.com>
MFC after: Right now