bsdinstall: Use pkgbase.f.o for BETA/RC/RELEASE

For BETA/RC/RELEASE builds, fetch files from the appropriate repository
on pkgbase.freebsd.org, using the appropriate signing keys. Note that
there is a separate repository for each BETA and RC; this ensures that
someone installing from e.g. a 15.0-RC1 ISO will get 15.0-RC1 bits and
not whatever happens to be the most recent build from releng/15.0.

Approved by: re (cperciva)
With hat: re
MFC after: 8 hours (needed in 15.0-RC1)

(cherry picked from commit 51184e38c932b7acedfc26fef3a0aad4c04f8959)
(cherry picked from commit 2ce97e800393057e3b8f041993678cb8af2c7607)