cmd/racebuild: tweak compilers used for windows syso build

Revise the recipe for building a windows race syso slightly to use
the existing compilers on the windows machine (in C:\godep\gcc64\bin)
as opposed to downloading GCC 5.X via "choco install". This requires
updating PATH following the refresh env.

Updates golang/go#35006.
Updates golang/go#53539.

Change-Id: I14c8491159f421f688f8d4b7c84250768d69ea42
Reviewed-on: https://go-review.googlesource.com/c/build/+/414475
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

cmd/racebuild/racebuild.go

@@ -237,9 +237,12 @@ cp llvm-project/compiler-rt/lib/tsan/go/race_netbsd_amd64.syso go/src/runtime/ra
 @"%SystemRoot%\System32\WindowsPowerShell\v1.0\powershell.exe" -NoProfile -InputFormat None -ExecutionPolicy Bypass -Command "[System.net.ServicePointManager]::SecurityProtocol = 3072; iex ((New-Object System.Net.WebClient).DownloadString('https://chocolatey.org/install.ps1'))" && SET "PATH=%PATH%;%ALLUSERSPROFILE%\chocolatey\bin"
 choco install git -y
 if %errorlevel% neq 0 exit /b %errorlevel%
-choco install mingw --version 5.3.0 -y
-if %errorlevel% neq 0 exit /b %errorlevel%
 call refreshenv
+echo adding back in compiler path
+set PATH=C:\go\bin;%PATH%;C:\godep\gcc64\bin
+rem make sure we have a working copy of gcc
+gcc --version
+if %errorlevel% neq 0 exit /b %errorlevel%
 git clone https://go.googlesource.com/go
 if %errorlevel% neq 0 exit /b %errorlevel%
 cd go