fix(build): sync go versions between promu crossbuild and docker images

[burningalchemist]

To have consistent builds we want to make sure Go version is the same for all the builds. It wasn't a problem before v1.22, but it seems to be a good opportunity to improve consistency these days.

Notable changes

• Update Dockerfile.multi-arch so it takes a build argument (with latest being a default value) that we can pick from .promu.yml;
• Add a step to GitHub actions to obtain the version from the config file, and store it in the step output key.

Additional notes

• In the future we might want to drop building binaries again and reuse ones built by promu crossbuild. So the Dockerfile will be changed, the builds also should become faster.