[pavelzw]

Description

Follow-up to #7193.

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Summary: would be nice to be able to self-update and get an unreleased pixi version from https://github.com/prefix-dev/pixi-nightly

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AI-generated details:

Adds pixi self-update --nightly, which updates to the latest development build of main from prefix-dev/pixi-nightly. --nightly --version 0.84.0-10-ge4b92dc9f installs a specific build.

• The latest-version lookup, release notes and download URL now come from a ReleaseChannel (Stable or Nightly). Before, --from-url only changed the download URL, so --from-url .../pixi-nightly/releases tried to download the latest stable tag from the nightly repo.
• --nightly conflicts with --from-url.
• Nightly versions (git describe output) parse as conda versions and sort as expected: 0.84.0 < 0.84.0-9-g… < 0.84.0-10-g… < 0.85.0. Unit tests cover this and the round-trip to the tag name.

Not done yet:

• pixi-nightly is still private, so this can't work end-to-end until it's public. I haven't run it against the real repo.
• A nightly build running plain pixi self-update still targets stable and asks whether to downgrade. It could default to the nightly channel when the current version is a nightly build.

Checklist:

• I've discussed the proposed changes with a maintainer.
• I understand the purpose of my changes and how they interact with the greater system.
• User-facing changes are documented and prose in the documentation has been written/edited/reviewed by hand.

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: Claude Code