common-updater-scripts: Only look for URL when replacing it
common-updater-scripts: Only look for URL when replacing it

This would do a pointless slow `nix-instantiate` call for a feature
that is rarely used – initially, it was added for Firefox updates
https://github.com/NixOS/nixpkgs/commit/186de9ca9ef5650180f2f0cf0a9efdc7dfb16e2e
but now not even that uses it.

Additionally, this would break stuff like `cargoDeps`,
which lack the url attributes.

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