scripts: system: Default to -v3 when building a system.

This is a followup to 8f9052d543.

* guix/scripts/system.scm (verbosity-level): Change the default
from 2 to 3 when building a system.

guix/scripts/system.scm

@@ -1145,7 +1145,7 @@ (define (verbosity-level opts)
   "Return the verbosity level based on OPTS, the alist of parsed options."
   (or (assoc-ref opts 'verbosity)
       (if (eq? (assoc-ref opts 'action) 'build)
-          2 1)))
+          3 1)))
 
 
 ;;;