release: make pkg installs more robust

Currently pkg(8) will fail to install any package if one is missing, so
make this a loop to prevent one missing package from preventing the rest
from installing. Seen building an AWS AMI for aarch64 on main and
ebsnvme-id is not available in the repo at the moment.

Approved by: gjb
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")