[cron] using latest revision instead of cache one

[iQQBot]

in cron job, we use a cached revision, which is not expected, we want use latest of target branch

This PR set revision to empty string, so that it can resolve by start time

https://github.com/csweichel/werft/blob/0fe8c76a992f76ba5936b4027fe78ed995b53a24/pkg/werft/service.go#L172-L177

https://github.com/csweichel/werft/blob/0fe8c76a992f76ba5936b4027fe78ed995b53a24/plugins/github-repo/pkg/provider/provider.go#L70-L78

https://github.com/csweichel/werft/blob/0fe8c76a992f76ba5936b4027fe78ed995b53a24/pkg/plugin/host/repo.go#L96-L108

[iQQBot]

@csweichel could you review this PR? 🙏