cmd/dist: reset version during bootstrap

Fixes #2927.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5651062

src/cmd/dist/build.c

@@ -1247,6 +1247,7 @@ cmdbootstrap(int argc, char **argv)
 		usage();
 
 	clean();
+	goversion = findgoversion();
 	setup();
 	
 	binit(&b);