Reduce provisioning / bootstrap time

Hi,

We're in the process of moving towards using lpcraft and Launchpad for various charms and other things. They're mostly configured to run lint checks as well as unit tests when a merge proposal is created and up for review.

With some things such as the canonical terraform plans, internal proxy config, and NGFW firewall configs, it runs small scripts mainly to check valid YAML and such. Unfortunately, the provisioning or bootstrap time of lpcraft jobs seems to take several minutes before the lpcraft configured things are started. An example being https://code.launchpad.net/~jsimpso/canonical-is-firewalls/+git/firewall-configs/+build/19803

I know there is LP:1995406 open to speed up APT package installs. In addition to this, maybe we can have an option to tell lpcraft to not run `apt-get update` and/or `apt-get dist-upgrade`?

Also, maybe there can be images lpcraft uses with python 3.x packages already installed?

Thanks!