Check out git branch before initializing bitbake

Stefan observed that CI job initializes bitbake with layer configuration
from the layer as checked out by git-repo, and only then switches to the
branch being tested.

This resulted in bitbake incorrectly failing to notice a layer that was
added by the tested patch.

The solution is to move all of the bitbake initialization stage from the
.build jobs "script_before" section, to the "script" section. In effect
any derivative job can use "script_before" to freely alter the source
tree before bitbake is initialized. Due to the lucky arrangement of YAML
anchors, the .build job's "script" keeps running with bitbake
environment sourced, so it can do exactly what one would expect - build
the desired recipe.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>