substitute: If a server's nar URL is 404, try the next one(s).

If a substitute server advertises in its narinfo, for example, both a
/zstd and a /lzip URL but the /zstd URL is unreachable, try the /lzip
URL.

Fixes <https://issues.guix.gnu.org/63634>.

* guix/narinfo.scm (narinfo-preferred-uris): New procedure.
(narinfo-best-uri): Rebase on top of it.
* guix/scripts/substitute.scm (download-nar)[try-fetch]: New procedure.
Use 'narinfo-preferred-uris' and 'try-fetch' to attempt all the URLs of
NARINFO.
* tests/substitute.scm (request-substitution): Remove 'parameterize'.
Delete DESTINATION.
("substitute, preferred nar URL is 404, other is 200"): New test.

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