There's no easy way to determine which version is younger or older. Tests in external repos sometimes need to be able to distinguish against which version of the std lib they are running; typically they require a certain "minimal" version.
There could be a runtime.VersionNumber() that returns some form of monotonically increasing value/string; or perhaps runtime.VersionTime() could return a value that is easily converted into a time.Time. Alternatively, runtime.Version() could always start with the most recent version, followed by the current version hash if not at a release. That would at least make it possible to discern against tagged releases. For instance: