cheribsd-release: include all built kernels

[nwf]

We already do this as part of installing the cheribsd-$ARCH targets, in BuildFreeBSD::_installkernel (called by BuildFreeBSD::install from BuildFreeBSD::process via delegation to its superclass Project::process), but BuildFreeBSDReleaseMixin overrides its process() method and does not delegate to this existing installation machinery.

Note that CheriBSD itself sets this NO_INSTALLEXTRAKERNELS control to "no" on aarch64c if it isn't already set, presumably so that self-hosted Morello builds do the right thing.