[Merge] ubuntu-release-upgrader:server-mode-check into ubuntu-release-upgrader:ubuntu/jammy

[Brian Murray]

Brian Murray has proposed merging ubuntu-release-upgrader:server-mode-check into ubuntu-release-upgrader:ubuntu/jammy.

Requested reviews:
  Julian Andres Klode (juliank)

For more details, see:
https://code.launchpad.net/~ubuntu-core-dev/ubuntu-release-upgrader/+git/ubuntu-release-upgrader/+merge/428312

Looking at the calls to self.cache.need_server_mode() in DistUpgradeController.py one of them can do more harm than good by switching from a "desktop" mode upgrade to a "server" mode upgrade. This changes that by only calling the function again if "server" mode is already set.

I tested an upgrade of an Ubuntu 20.04 server install with this change and it worked without issue. Additionally, I tested a desktop upgrade of Ubuntu 22.04 (only the calculation of the DistUpgrade) and it also worked without an issue.

It might also be necessary to reset `os.environ["RELEASE_UPGRADE_MODE"]`, however it looks like there is an existing bug with it not being set after changing mode.
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