shell: Don’t rely on atime for caching.

Relying on atime was fine even on ‘noatime’ file systems because the atime was
explicitly updated.  However, on file systems with atime enabled, cache items
were potentially retained for too long since scanning the cache directory
could update atimes.

* guix/scripts/shell.scm (options-with-caching): In ‘utime’ call, change both
the mtime and the atime.
(guix-shell)[entry-expiration]: Use ‘file-expiration-time’ with ‘stat:mtime’.

Change-Id: Ibe2aedae41a1018a95c2efe7f2dcf08bd1caae2a

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