env: Fix so built shipped/provided packages are Pinned

[wdoekes]

Description

Once an image has been created it is still writable: apt/dpkg works. This is nice. We can install additional tooling.

However, there is a problem with the versioning of certain provided packages:

• a bash-5.1 is built, but it has the same name/version as the upstream original
• additionally, a newer bash might be available and we can apt-get upgrade to it (breaking any specifics patches added here)

A solution:;

• a good start could be to also update the changelog with a new version (<upstream_version>+sonic)
• and then we'd like to Pin the packages;
  • this could be done through a repository -- which is a PITA, but
  • it could also be done through a local repository (file-based, like a cdrom). and the files do not need to be there, as long as the Pin-prio can get high enough that you cannot accidentally replace the package

Maybe check:
https://askubuntu.com/questions/170348/how-to-create-a-local-apt-repository

Which build are we running (if any)

Upstream issues/PRs

[wdoekes]

Looks like we need:
/etc/apt/preferences.d/sonic.pref

With this content:

Package: bash
Pin: version 5.1-2
Pin-Priority: 800

...

• Priority < 0: prevents the installation of a package version
• Priority value between 1 and 99: causes a version to be installed only if there is no installed version of the package.
• Priority between 100 and 499: causes a version to be installed unless there is a version available belonging to some other distribution or the installed version is more recent
• Priority between 500 and 989: causes a version to be installed unless there is a version available belonging to the target release or the installed version is more recent
• Priority between 990 and 999: causes a version to be installed even if it does not come from the target release, unless the installed version is more recent
• Priority > 1000: causes a version to be installed even if this constitutes a downgrade of the package.

With this, we won't even need #deb [trusted=yes] file:/usr/local/mydebs ./, I think.

Needs confirmation when two equal-named packages are available..