CI: set GLUON_BRANCH depending on build type

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        |