ci: Use `.kokkuri:remote-context`

The latest version of Kokkuri (v2.2.0) now defines an extendable
.kokkuri:remote-context job which sets up the build context variable
in the same way that was done here ad hoc.

Remove BUILD_FRONTEND from .gitlab-ci.yml. The new version of
Kokkuri can now resolve the frontend from remote config contexts hosted
on GitLab.

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