srcOnly: ignore additional arguments

This reverts commit b32a0574,
which breaks even the most straightforward uses of srcOnly:

    nix-repl> srcOnly guile
    error: anonymous function at /home/src/nixpkgs/pkgs/build-support/src-only/default.nix:1:1 called with unexpected argument 'drvPath', at /home/src/nixpkgs/lib/customisation.nix:69:16

    nix-repl> srcOnly hello
    error: anonymous function at /home/src/nixpkgs/pkgs/build-support/src-only/default.nix:1:1 called with unexpected argument 'drvPath', at /home/src/nixpkgs/lib/customisation.nix:69:16

Link: https://github.com/NixOS/nixpkgs/pull/80903#issuecomment-617172927