ci: Refactor the CI pipeline

ci: Refactor the CI pipeline

We should have a single job for building both static and shared
versions of GTK. On the other hand, having a separate job for the
release build would be a plus.

Additionally, we shouldn't use an opaque script to build GTK; the only
step in the process that requires hand-holding is running the tests
suites under Xvfb, and having the build options visible from the YAML
file gives us a better idea of what kind of build we're running.