Auto merge of #29621 - mukilan:upload-nightly-to-gh-releases, r=mrobinson

Upload nightly builds to Github Releases

This change extends the `mach upload-nightly` command to publish the nightly builds for all platforms as GH Release assets.

The GH releases are made on a separate repository so that we can persist older nightly builds without having to accumulate git tags for them.

Example releases on a fork can be seen here: https://github.com/mukilan/servo-nightly-builds/releases/
Assets of the *latest* nightly releases will available at a stable url e.g for linux
`https://github.com/mukilan/servo-nightly-builds/releases/latest/download/servo-latest.tar.gz`

Some design tradeoffs in this approach are:
1. To allow the 'latest' link from servo.org to remain stable, the release assets are named 'servo-latest.{ext}' instead of containing the release tag/date.
2. The release is created as draft and published atomically when all platforms have been built successfully. This allows us to link to the 'latest' alias from servo.org while gauranteeing that it contains builds for all platforms. The other option here would be to have code in servo.org UI that uses GH API to find the most recent release with a successful build for a given platform.
3. The tags in the nightly repo are all based on the same commit that has no relation to servo code base.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

<!-- Either: -->
- [ ] There are tests for these changes OR
- [x] These changes do not require tests because it extends the nightly CI job

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