Add support for skipping the bot

[tresf]

Some PRs don't need a download, such as LMMS/lmms#4884. It would be nice to have the PR bot ignore certain builds.

Note @lukas-w also mentioned that the CI can be avoided directly by adding the keyword [skip ci] to the commit message, but I don't know who'd be thinking of that at PR time.

[Reflexe]

:)
Fixed.
use the bot-skip label for that.

[tresf]

Closed by 87687b3. (added for completeness sake)

[Reflexe]

Update: recommited without the mistake.