Github API uses versions where it should use tags

While working on #386 I noticed that at least in th Github V3 API we use a version where a tag should be used.

The relevant bit of code
https://github.com/ocamllabs/dune-release/blob/a029ffe0fdd84b34953c315f6298d6ef1942f395/lib/github_v3_api.ml#L58-L61

Thus I have the suspicion that this breaks in cases where the version and the tag do not match, e.g. if the version contains a `~`.