dhall: Use --base-import-url flag for documentation (#128588)
This updates `pkgs.dhallPackages.buildDhallGitHubPackage` to use the
newly added `--base-import-url` `dhall-docs` flag. This flag is used
by the generated documentation so that paths copied to the clipboard
represent complete URLs that can be imported instead of only the
relative path to the import.
authored by Gabriel Gonzalez and committed by GitHub ef0a4234 c865d036