Reported by Christopher Baines <guix@cbaines.net>.
* guix/cache.scm (maybe-remove-expired-cache-entries): Define
‘expiry-port’; create it with ‘lock-file’. Change ‘last-expiry-date’
accordingly. Write timestamp straight to ‘expiry-port’.
* tests/cache.scm ("maybe-remove-expired-cache-entries, cleanup needed
but lock taken"): New test.
Change-Id: I22441d9d2c4a339d3d3878de131864db5a0ae826