substitute: Don’t keep cache entries more than a few days.

Experience has shown that keeping too many entries increases disk usage
and, more importantly, leads to long delays when cleaning up the cache,
measured in minutes on slow or busy HDDs with hundreds of thousands of
cache entries, as is common on build machines.  In those cases, the cost
of the cache outweighs its benefit.

* guix/scripts/substitute.scm (%narinfo-expired-cache-entry-removal-delay):
Reduce to 5 days.
(cached-narinfo-expiration-time)[max-ttl]: Reduce to 2 days.

Change-Id: Iab212f572ee9041be61716423a3c014f93fe81ed

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