env/windows: more fixes to test_buildlet.bash

Fix typo in URL for Go download. Add dummy VERSION to Go repo before
uploading to windows builder.

Updates golang/go#35006

Change-Id: I832aa781884b4da99053bd793909a693900fa937
Reviewed-on: https://go-review.googlesource.com/c/build/+/413414
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Alex Rakoczy <alex@golang.org>
Run-TryBot: Than McIntosh <thanm@google.com>

env/windows/test_buildlet.bash

@@ -18,7 +18,8 @@ echo "Pushing go tip to buildlet"
 (
   TEMPDIR=`mktemp -d `
   cd $TEMPDIR
-  git clone //go.googlesource.com/go
+  git clone https://go.googlesource.com/go
+  echo "devel.dummy_to_avoid_git_usage" > go/VERSION
   tar zcf - ./go > go.tar.gz
   gomote puttar "$BUILDLET" go.tar.gz
   rm -rf go go.tar.gz