gst-plugins-rs: version mismatch is error only when tagged_for_release

Checking for origin/main is not enough, because once we start making development releases, gst-plugins-rs starts tracking a stable branch of gst-plugins-rs as soon as it becomes available. A version change in there means that we start failing CI for no reason.

We only need to update the version when tagging for a release, at which time we will affix to a tag. In order to check that condition, we need to move the check to the recipe.