ci: add bot to build on comment

ci: add bot to build on comment

This has been requested many times, so let's add this to speed up reviews. When
a member of the "reviewers" group comments the magic word written below, that
specific firmware is created and attached by a bot.

    build <target>/<subtarget>/<profile>

Members of the "reviewers" group have no extra privileges, they can not commit
to the repository nor perform any action outside the `build-on-comment` action.

Motivation is to speedup reviews and have a better source for sharing compiled
firmware.

Signed-off-by: Paul Spooren <mail@aparcar.org>