substitute: Work around Guile 2.2 'time-monotonic' bug.

Prior to this change, half of the cached narinfos would expire
immediately since they contained the number of nanoseconds instead of
the number of seconds as their date.

* guix/scripts/substitute.scm (time-monotonic) <guile-2.2>: Define, as a
workaround.

guix/scripts/substitute.scm

@@ -96,6 +96,13 @@
 ;;;
 ;;; Code:
 
+(cond-expand
+  (guile-2.2
+   ;; Guile 2.2.2 has a bug whereby 'time-monotonic' objects have seconds and
+   ;; nanoseconds swapped (fixed in Guile commit 886ac3e).  Work around it.
+   (define time-monotonic time-tai))
+  (else #t))
+
 (define %narinfo-cache-directory
   ;; A local cache of narinfos, to avoid going to the network.  Most of the
   ;; time, 'guix substitute' is called by guix-daemon as root and stores its