Remove double `v` in Nix update

[Baltoli]

This is just a simple typo that was producing an error:

error: unable to download 'https://api.github.com/repos/runtimeverification/k/commits/vv7.0.31': HTTP error 422

       response body:

       {
         "message": "No commit found for SHA: vv7.0.31",
         "documentation_url": "https://docs.github.com/rest/commits/commits#get-a-commit"
       }