sled postboot service should not be included by default

I was rebuilding a Gimlet image with the following command:

ptime -m ./helios-build image \
    -p helios-dev=https://pkg.oxide.computer/helios/2/dev/ \
    -P "$proto" \
    -B \
    -F optever=0.36 \
    -F ntp \
    -N "$name" \
    -o "$OSDIR/$image"

The image didn't end up working the way I expected, because it included a svc:/site/postboot service that conflicted with the definition of the one I layer in via the external proto area facility (-P).

It occurs to me that this service should not be included unless explicitly requested for bench Gimlet images, as it does things we do not want production sleds to do; e.g., importing a ZFS pool called "data" if one exists.

Start at the ./helios-build image entry point and trace how the default svc:/site/postboot service is selected, including the external proto area facility passed with -P. Compare the generated image with and without that service. Done means postboot is absent by default and included only when explicitly requested, without importing the production data pool.