Just for cross-reference, here is the bug reported for this:
That could be a useful feature to add. But in your case, I think it would be better to have a consistent valac version, no matter where the snap is build. You can achieve that by adding a part for vala in your snapcraft.yaml and compiling it during the build. Or even better, make and publish a valac snap, and use it as one of the
build-snaps in your snapcraft.yaml.