release: Make toolchain OCI container use pkg set

This also indirectly addresses the lack of FreeBSD-lldb package on
the riscv64 platform by relying on that package being part of the
FreeBSD-set-devel set iff it exists.

A subsequent commit will adjust the "notoolchain" OCI container image
to also use a package set, which will indirectly affect the contents
of the "toolchain" container (which is an explicit superset of the
"notoolchain" container).

MFC after: 1 day
Co-authored-by: dch