channels: Do not fail when the inferior lacks 'guix repl'.

channels: Do not fail when the inferior lacks 'guix repl'.

Fixes <https://bugs.gnu.org/34637>.
Reported by Martin Flack <martin.flack@gmail.com>.

Previously we'd fail to build the package cache for old versions of Guix
that lack 'guix repl'.  Now we simply ignore the issue and keep going
without a cache.

* guix/inferior.scm (gexp->derivation-in-inferior): Add
 #:silent-failure? and honor it.
[drop-extra-keyword]: New procedure.
Use it.
* guix/channels.scm (package-cache-file): Pass #:silent-failure? #t.