Streamline interactive widget testing in the Lab

While the automated tests that run as part of the test build job are sufficient for testing most libraries, there are certain tools that require interactive testing in the Lab environment itself, such as the TVB widgets.

To have this, we need to create a new kernel (and a new image, if needed) in the integration environment of the EBRAINS Lab that will act as a testing environment for fixes/new features.

The flow will be implemented as follows:

• adapt CI so that any branch named lab-<feature> is deployed in a new Spack environment/kernel in the lab-int
• create such a branch for widgets, e.g. lab-widgets
• merge any changes to the widget packages there first and test in the lab-int
• after testing, merge to master
• (if possible) allow widgets maintainers to merge to lab-widgets