build: add runner parameter
This allows to configure which run image to use in preparation to being able to use different ones (ex. ubuntu 22/24).
This helps also when upgrading/changing the tags of the builder and runner, as currently the runner is configured in the builds-builder side, and the builder in builds-api, this moves that config to builds-api so we only need changes in one repo upgrade both
Bug: T380127
Signed-off-by: David Caro dcaro@wikimedia.org