ci: add release job

[benoit-pierre]

Run on tag.

Because download stuff from git.savannah.gnu.org is so flaky:

• the release job runs even if some toolchain jobs failed (but not on cancellation)
• a draft release is created

This way it's possible to re-run failed jobs until all artifacts have been uploaded to the release.

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This change is 

[Frenzie]

Sounds good. What about the failed jobs in this PR?

[benoit-pierre]

That's the git.savannah.gnu.org failures in action:

Done. You may now run 'ct-ng menuconfig' to edit the configuration.
  MKDIR scripts
  DOWNLOAD  scripts/config.sub

Failing to download http://git.savannah.gnu.org/gitweb/?p=config.git;a=blob_plain;f=config.sub;hb=HEAD.

[benoit-pierre]

Can you tag a new release?

[Frenzie]

https://github.com/koreader/koxtoolchain/actions/runs/14946424759

[benoit-pierre]

Don't forget to publish the draft ;)