Motivation for this change

nixos-unstable is currently blocked. See https://hydra.nixos.org/build/126501646 for an example of a failing build.

To reproduce the failure, run:

nix-build pkgs/top-level/release.nix -A unstable --show-trace

The failing call to nix-instantiate can be seen here.

This fails, because find-tarballs.nix expects the url argument to fetchurl to be a string, while the urls should be used when multiple sources are needed.

This commit fixes a single instance of this problem, and this fix seems to be enough to unblock nixos-unstable. I am also preparing a follow-up PR to fix all additional instances of the problem.

CC: @jonringer

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.