store: Memoize 'built-in-builders' call directly in <store-connection>.

The caching strategy introduced in
40cc850aebb497faed0a11d867d8fcee729023df was ineffective since we
regularly start from an empty object cache.  For example, "guix build
inkscape -n" would make 241 'built-in-builders' RPCs.

* guix/store.scm (<store-connection>)[built-in-builders]: New field.
(open-connection): Adjust '%make-store-connection' call accordingly.
(port->connection): Likewise.
(built-in-builders): Rename to...
(%built-in-builders):  ... this.
(built-in-builders): New procedure.
* guix/download.scm (built-in-builders*): Remove 'mcached' call.

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