run: Allow git tag as ref

[geropl]

Is your feature request related to a problem? Please describe.
Every time I trigger werft run github org/repo:ref ... I expect it to take a tag as ref but it complains with:
Error: rpc error: code = Internal desc = GET https://api.github.com/repos/...: 404 Branch not found []

Describe the solution you'd like
Accept tags.

Describe alternatives you've considered
I can use the branch at whose HEAD the tag points to, but that's not always 100% the same.

Additional context