cmd/dist: reflect local changes to tree in goversion
cmd/dist: reflect local changes to tree in goversion

runtime.Version() requires a trailing "+" when
tree had local modifications at time of build.

Fixes issue 7701

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://codereview.appspot.com/84040045

Committer: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang