srcOnly: ignore additional arguments
srcOnly: ignore additional arguments

This reverts commit b32a057425b337b37db41d3b4a67d39657477c05,
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

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