Build the manifest for stable, beta and nightly in the package step.
This avoids the need of a rebuild to publish the firmware on a
different channel.

Just always default to the stable autoupdater branch. Just because
someone uses a nightly or beta image doesn't necessarily mean we
should also push them to that update channel.  If a user wants to
change, they can still do so in config mode.

see <https://blog.sparksuite.com/7-ways-to-speed-up-gitlab-ci-cd-times-29f60aab69f9>

Build cache is kept per build target. Each tag also has a seperate build
cache, so tagged builds are always built from scratch.

Introduce the following build type:
| Build Type | When   |
|------------|--------|
| stable     | commit tag starts with "stable/" |
| beta       | commit tag starts with "beta/" |
| nightly    | trigger is a scheduled job |
| commit     | else |

Depending on the build type, set the GLUON_BRANCH accordingly:

| Build Type | GLUON_BRANCH  |
|------------|---------------|
| commit     | (unset)       |
| nightly    | nightly       |
| beta       | beta          |
| stable     | stable        |

When building non-tagged and non-scheduled commits, only build
x86-64 to speed up the build.