Set helper_image_flavor to ubi-fips when fips mode is enabled

#38273 (closed) describes a situation where jobs fail when runner is fips enabled and running in so-called "air-gapped" environments. The failure occurs because runner tries to pull the helper image, but the pull fails because there's no access to the internet.

The thing is, the ubi-fips-flavoured image already exists in the FS as an exported image, and we want to (are meant to) use that helper image! All we have to do for this to happen is to set the helper_image_flavor top ubi-fips, and runner will do the right thing.

Initially I thought to set this when running the install command, but this is an executor-specific setting, and at that time there is are configured runners. The right time to do this is on runner registration.