RFE: add ability to skip scratch builds / CI

[michel-slm]

Description

It would be nice to have the ability, when filing a PR, to request that scratch builds and the rest of CI to be skipped. This is necessary when e.g. submitting stacked PRs where the testing typically happens in a separate COPR anyway, otherwise the build will just fail and we're wasting resources.

Benefit

Efficiency win. We're no longer throwing scarce compute / IO resources at buillds that are known in advance that they will fail

Importance

Medium - it both wastes resources and also a reviewer that's not careful might just close or ignore the PR because "it failed to build'

What is the impacted category (job)?

Copr builds

Workaround

• There is an existing workaround that can be used until this feature is implemented.

Participation

• I am willing to submit a pull request for this issue. (Packit team is happy to help!)

[LecrisUT]

Probably this would need reading a pre-defined label on pagure/fogejo. Suggestion for the label name? We could also use a regex pattern to make the label slightly configurable?