guix build: Only 'guix build' sets #:print-build-trace.

This makes 'guix package' et al. slightly less verbose, by not emitting
"@ substituter-started" lines and similar.

* guix/scripts/build.scm (set-build-options-from-command-line): Pass
  #:print-build-trace.
  (%default-options): Add 'print-build-trace?'.