challenge: Support "--diff=diffoscope".

challenge: Support "--diff=diffoscope".

* guix/scripts/challenge.scm (call-with-nar): New procedure.
(narinfo-contents): Express in terms of 'call-with-nar'.
(call-with-mismatches, report-differing-files/external): New
procedures.
(%diffoscope-command): New variable.
(%options): Support "diffoscope" and a string starting with "/".
* tests/challenge.scm (call-mismatch-test): New procedure.
("differing-files"): Rewrite in terms of 'call-mismatch-test'.
("call-with-mismatches"): New test.
* doc/guix.texi (Invoking guix challenge): Document it.