Support installing without a network connection

[TraceyC77]

There may be cases where a user doesn't have access to a network and would like to install Solus
Allow installing without network access

[TraceyC77]

We currently use networking to download the eopkg repo
This is a workaround for ISOs
@EbonJaeger has noted that we used to allow installation without network
This may require changes in ISO tooling.

[EbonJaeger]

To expand on the above, we added a couple of steps to the post-install phase of installation with Calamares as safety rails in case anyone tried to release ISOs with the Unstable repository instead of Shannon. I think the new ISO tooling also sets the stable repository, as long as you're not building a smoketest version.

The downside of also doing this as a post-install step is that users are now required to have a working internet connection to install Solus. But, we want users to still be able to install without an internet connection. So, the way I see it, we have two options:

1. Move the post-install process to a contextual process where we should be able to check for internet connectivity. I asked about this before in the Calamares Matrix room, and they indicated that there should be a global storage key for it. We could also move some or all of the current steps into a script that is then run from the current post-install step, and check for connectivity in the script.
2. Or, we could just rely on the ISO tooling, and not worry about this in Calamares at all.