build: restore PATH before "dist banner"

        Fixes #3699.

R=golang-dev, r, rsc
CC=golang-dev
https://golang.org/cl/6272048

src/all.bash

@@ -8,6 +8,8 @@ if [ ! -f make.bash ]; then
 	echo 'all.bash must be run from $GOROOT/src' 1>&2
 	exit 1
 fi
+OLDPATH="$PATH"
 . ./make.bash --no-banner
 bash run.bash --no-rebuild
+PATH="$OLDPATH"
 $GOTOOLDIR/dist banner  # print build info

src/all.bat

@@ -11,10 +11,12 @@ echo all.bat must be run from go\src
 goto end
 :ok
 
+set OLDPATH=%PATH%
 call make.bat --no-banner --no-local
 if %GOBUILDFAIL%==1 goto end
 call run.bat --no-rebuild --no-local
 if %GOBUILDFAIL%==1 goto end
+set PATH=%OLDPATH%
 go tool dist banner
 
 :end