pull: Add missing monadic 'return'.

pull: Add missing monadic 'return'.

Fixes a regression introduced in
5f7dd092ca577a534067f577b8849ed06cabf970 where, upon completion, 'guix
pull' would fail (instead of printing the new/upgraded packages) with
ugly errors like:

  successfully built /gnu/store/…-profile.drv
  1 package in profile
  Backtrace:
  […]
  In guix/store.scm:
    1605:24  1 (run-with-store _ _ #:guile-for-build _ #:system _ # _)
  In unknown file:
	     0 (_ #<build-daemon 256.97 2476b40>)

  ERROR: Wrong type to apply: #t

Reported by thorwil on #guix.

* guix/scripts/pull.scm (build-and-install): Add missing 'return' when
DRY-RUN? is wrong.