[Github][CI] Upload artifacts directory for premerge workflow

The premerge pipeline currently creates an artifacts directory with some
statistics that gets uploaded on the buildkite side for later
inspection. This patch adds support for this on the Github side by using
the upload artifacts action.

Reviewers: Keenuts, lnihlen, mizvekov, tstellar, Endilll

Reviewed By: mizvekov

Pull Request: https://github.com/llvm/llvm-project/pull/135538