hosts.{kiwi,plum}: Use `forgejo-runner` for CI runners.

Also added runner to trusted users although I'm not sure it's the best approach. I don't know if it even gives it the necessary access to things like trusted-public-keys despite what the warnings in CI show.

Created by  PlumJam  on November 11, 2025
TF7L7GT4UDTRC4YFK4RBXRJYK5SNTPNGDFOPLLKLEDK27GA3GAEQC
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