build: fix race.bat flags

The old C-based dist accepted merged flags (-wp) but the Go-based dist
requires -w -p

This should get the Windows race builder running properly, along with
https://go-review.googlesource.com/#/c/4132/

Update #8640

Change-Id: Ic17bbe9ea6c8b3d3e9b29f94e234d014f2926439
Reviewed-on: https://go-review.googlesource.com/4133
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/race.bat

@@ -18,7 +18,7 @@ goto end
 set GOROOT=%CD%\..
 call make.bat --dist-tool >NUL
 if errorlevel 1 goto fail
-.\cmd\dist\dist env -wp >env.bat
+.\cmd\dist\dist env -w -p >env.bat
 if errorlevel 1 goto fail
 call env.bat
 del env.bat