libfetchers/github: allow `url` attribute

Since 108debef we allow a
`url`-attribute for the `github`-fetcher to fetch tarballs from
self-hosted `gitlab`/`github` instances.

However it's not used when defining e.g. a flake-input

    foobar = {
        type = "github";
        url = "gitlab.myserver";
        /* ... */
    }

and breaks with an evaluation-error:

    error: --- Error --------------------------------------nix
    unsupported input attribute 'url'
    (use '--show-trace' to show detailed location information)

This patch allows flake-inputs to be fetched from self-hosted instances
as well.

src/libfetchers/github.cc

@@ -77,7 +77,7 @@ struct GitArchiveInputScheme : InputScheme
         if (maybeGetStrAttr(attrs, "type") != type()) return {};
 
         for (auto & [name, value] : attrs)
-            if (name != "type" && name != "owner" && name != "repo" && name != "ref" && name != "rev" && name != "narHash" && name != "lastModified")
+            if (name != "type" && name != "owner" && name != "repo" && name != "ref" && name != "rev" && name != "narHash" && name != "lastModified" && name != "url")
                 throw Error("unsupported input attribute '%s'", name);
 
         getStrAttr(attrs, "owner");