emacs: Disable build traces produced by the daemon.

* emacs/guix-main.scm (process-package-actions,
  package-source-build-derivation): Pass #:print-build-trace #f to
  'set-build-options', to reduce verbosity.

emacs/guix-main.scm

@@ -857,6 +857,7 @@ (define* (process-package-actions
                (derivations (list derivation))
                (new-profile (derivation->output-path derivation)))
           (set-build-options store
+                             #:print-build-trace #f
                              #:use-substitutes? use-substitutes?)
           (show-manifest-transaction store manifest transaction
                                      #:dry-run? dry-run?)
@@ -908,6 +909,7 @@ (define* (package-source-build-derivation package-id #:key dry-run?
       (let* ((derivation  (package-source-derivation store source))
              (derivations (list derivation)))
         (set-build-options store
+                           #:print-build-trace #f
                            #:use-substitutes? use-substitutes?)
         (show-what-to-build store derivations
                             #:use-substitutes? use-substitutes?