substitute: Ignore irrelevant narinfo signatures.

substitute: Ignore irrelevant narinfo signatures.

Fixes <https://bugs.gnu.org/33733>.

Fixes a bug whereby 'guix substitute' would accept narinfos whose
signature does not cover the StorePath/NarHash/References tuple.

* guix/scripts/substitute.scm (narinfo-sha256)[%mandatory-fields]: New
variable.
Compute SIGNED-FIELDS; return #f unless each of the %MANDATORY-FIELDS
is among SIGNED-FIELDS.
 * tests/substitute.scm ("query narinfo with signature over nothing")
("query narinfo with signature over irrelevant bits"): New tests.