Don't leave the default set of archive servers

[nobuto-m]

Follow-up of c39bef1.

If we don't disable it, the system uses both the default set of servers
and the mirror specified. It's not so harmful in fully online scenarios,
but it causes issues in air-gapped or restricted environments. By
leaving the default servers, the retrofitting process has to wait for
timeout for each component like DNS timeout for archive.ubuntu.com and
it makes the process longer than usual like from a few minutes to hours
unnecessarily.

Also, the step to disable those servers must be 02- or earlier since
03-baseline-tools already uses APT so the original 42- position was too
late. Furthermore, putting multi-line content into DIB_UBUNTU_MIRROR is
not ideal but it's necessary to keep the backward compatibility. It was
nicer if we leveraged DIB_DISTRIBUTION_MIRROR in
diskimage_builder/elements/ubuntu/pre-install.d/01-set-ubuntu-mirror and
DIB_DEBIAN_COMPONENTS in the first place.

Closes-Bug: https://launchpad.net/bugs/2011527

[nobuto-m]

03-baseline-tools also uses apt-get update basically so we need to set the mirror earlier than the current phase.

[wolsen]

Hmm, it would be ideal to use the DIB_DISTRIBUTION_MIRROR and DIB_DEBIAN_COMPONENTS original (as requested in the review for pull request #20). However, it has its own limitations as the ubuntu elements don't support the DIB_DEBIAN_COMPONENTS option.

I think the right design would probably be to get away from the oneline apt sources format and move to the DEB 822 format (introduced in apt 1.1 so any modern Ubuntu has it). This would allow us to add a .sources file instead of a .list file and specify the Types, URIs, Suites (aka pockets), Components and Options separately which I think is easier to configure from a snap and/or charm perspective.

That seems like a lot of refactoring, and this solution may be okay for the short term while switching to deb822 format should allow a migration path that allows for the multiline solution (which really a multi-suites and components option).