How can I unpack the tarball?  The following returns "In execvp of tar:
No such file or directory."

(arguments `(#:modules ...
			 #:builder
			 (begin
			   ...
			   (let* ((tarball (assoc-ref %build-inputs "tarball"))
					  ...)
				 (system* "tar" "xvf" tarball)
				 ...))))

(inputs `(...
		  ("tarball" ,source)))

(display tarball) returns '/nix/store/6xwqfw5k58l0bhzzm8s81rrkmhm88vk4-grue-hunter.tar.gz'.