Using `_` is custom domain alias fails

[Vadorequest]

Describe the bug

##[debug]VERCEL_ALIASES_FAILED_FULL='[{"error":{"code":"invalid_alias","message":"The supplied alias nrn-v2-mst-aptd-at-lcz-sty-c1-fix-usage-is_server_initial_build.vercel.app is not a valid hostname. Make sure it excludes http:// and slashes"}}]'

Alias auto-created from branch, the branch name was fix-usage-IS_SERVER_INITIAL_BUILD.

Using a _ in branch name yields an error because using a _ in a domain name is not valid.
_ should be converted to -.