download: Work around Guile 2.2 bug with 'time-monotonic' objects.

* guix/build/download.scm (time-monotonic) [guile-2.2]: New variable.

guix/build/download.scm

@@ -140,6 +140,14 @@ Otherwise return STORE-PATH."
                        (string-drop base 32)))
       store-path))
 
+(cond-expand
+  (guile-2.2
+   ;; Guile 2.2.0 to 2.2.2 included 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* (progress-proc file size
                         #:optional (log-port (current-output-port))
                         #:key (abbreviation basename))