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

Fixes <http://issues.guix.gnu.org/55638>.

* guix/cache.scm (maybe-remove-expired-cache-entries)[last-expiry-date]:
Use 'get-string-all' + 'string->number' instead of 'read'; ignore
invalid numbers.
* tests/cache.scm ("maybe-remove-expired-cache-entries, empty cache")
("maybe-remove-expired-cache-entries, corrupted cache"): New tests.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>

This is a followup to e688c2df3924423b67892cc9939ca099c729d1cb.

* build-aux/hydra/evaluate.scm <top level>: Remove 'time-monotonic'
definition.
* guix/cache.scm: Likewise.
* guix/progress.scm: Likewise.
* guix/scripts/substitute.scm: Likewise.
* guix/scripts/weather.scm: Likewise.
* tests/cache.scm: Likewise.

* guix/cache.scm (time-monotonic) [guile-2.2]: New variable.
* tests/cache.scm (time-monotonic) [guile-2.2]: Likewise.
* guix/build/download.scm (time-monotonic) [guile-2.2]: Adjust comment:
it's a 2.2.2 bug.

* guix/cache.scm, tests/cache.scm: New files.
* Makefile.am (MODULES, SCM_TESTS): Add them.
* guix/scripts/substitute.scm (obsolete?): Remove.
(remove-expired-cached-narinfos): Rename to...
(cached-narinfo-expiration-time): ... this.  Remove the removal part and
only keep the expiration time part.
(narinfo-cache-directories): Add optional 'directory' parameter and
honor it.
(maybe-remove-expired-cached-narinfo): Remove.
(cached-narinfo-files): New procedure.
(guix-substitute): Use 'maybe-remove-expired-cache-entries' instead of
'maybe-remove-expired-cached-narinfo'.