src/make.bat: add missing go.exe extension

Got error:
'go' is not an internal or external command, nor is it a runnable program

Change-Id: Ie45a3a12252fa01b67ca09ef8fbb5b4bbf728fe7
GitHub-Last-Rev: 451815cacd9bfc509fa0aab3be54303797e605a2
GitHub-Pull-Request: golang/go#27214
Reviewed-on: https://go-review.googlesource.com/131397Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/make.bat

@@ -77,7 +77,7 @@ set GOROOT=%GOROOT_BOOTSTRAP%
 set GOOS=
 set GOARCH=
 set GOBIN=
-"%GOROOT_BOOTSTRAP%\bin\go" build -o cmd\dist\dist.exe .\cmd\dist
+"%GOROOT_BOOTSTRAP%\bin\go.exe" build -o cmd\dist\dist.exe .\cmd\dist
 endlocal
 if errorlevel 1 goto fail
 .\cmd\dist\dist.exe env -w -p >env.bat