Only Release Affected Runtimes

[joepetrowski]

Some releases (e.g. 1.2.1) only release certain runtimes, which one can see from the runtime version. When a release is triggered automatically, it should only include runtimes that have the corresponding version.

[bkchr]

For that to work we should base the decision which runtimes to release based on the spec_version. If that isn't bumped, a runtime is not going to be released.