feat: Add "Test Keyboards" link to all PRs

[ermshiperete]

Currently the testbot adds the "Test Keyboards" test artifacts link only for PRs where the keyboards get build, but not for Linux (don't know about other platforms).

It would be helpful if the "Test Keyboards" would always be created. In case where the keyboards don't get build because the TC job doesn't run we could link the latest build of the base branch.

[mcdurdin]

We'll do this as a part of keymanapp/keyman#5815