layers/config: use SingleBranchScheduler for PRs
layers/config: use SingleBranchScheduler for PRs

since AnyBranchScheduler appears to be creating
builds on multiple branches, rather than just the
targeted branch.

Signed-off-by: Matt Madison <1fa2ef4755a9226cb9a0a4840bd89b158ac71391@madison.systems>

madisongh/autobuilder