Skip release steps that already have been done

[Kalmalyzer]

We should ensure that releases can be re-run for a given version number.

For example, if the apt packaging step fails due to cosmic rays, we should be able to re-run the entire build/release process and have the apt package re-built & published.

Also, in the future, if we introduce a new publishing method, we should be able to re-run the entire build/release process and have the new publishing method get populated, without having to delete the current release, or having to pretend there is a new version.

• [x] If the GitHub release already has been created, skip that step.
• [x] Only upload those GH release artifacts that are missing.
• [ ] Re-publishing a package with the same version number to apt should be OK without error
• [ ] Re-bumping the formula to the same URL/version should be OK without error

[Kalmalyzer]

We should probably have the following mechanisms for triggering builds:

• remote dispatch event for building nightly
• remote dispatch event for updating config + building release
• remote dispatch for re-building the currently-configured release
• push tag for updating config + building release

[Kalmalyzer]

Remote dispatch events for re-building the currently-configured release is not a good idea. The build job is then launched without any associated tag. That makes some operations (like creating a GH release) fail.

It's probably better to re-run a release build by deleting and re-adding the release tag (case 4 with some extra glue logic).

[Kalmalyzer]

Ok, so GitHub releases are handled better now thanks to #103 . Not sure how the apt & homebrew publishing steps fare; let's leave that for later.