For #21396
# Details
This PR updates the automated release cycle for Orbit desktop, so that
it triggers based on a pushed _tag_ rather than a pushed PR. This has
the following benefits:
* The release can be based off of any branch, rather than always using
`main` as the base, so we can safely do patch release of desktop without
including in-progress code from main
* It brings the desktop release process more in line with the main Orbit
release process -- both are now triggered by a tag push.
We still create a PR for the release, to include a changelog.
# Checklist for submitter
If some of the following don't apply, delete the relevant line.
<!-- Note that API documentation changes are now addressed by the
product design team. -->
- [ ] Manual QA must be performed in the three main OSs, macOS, Windows
and Linux.
- [ ] Auto-update manual QA, from released version of component to new
version (see [tools/tuf/test](../tools/tuf/test/README.md)).
## Testing
To do -- will discuss with @lucasmrod
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Co-authored-by: Luke Heath <luke@fleetdm.com>
