go.cmd lets you run just "./tool/go" on Windows the same as Linux/Darwin.
The batch script (go.md) then just invokes PowerShell which is more
powerful than batch.
I wanted this while debugging Windows CI performance by reproducing slow
tests on my local Windows laptop.
Updates tailscale/corp#28679
Change-Id: I6e520968da3cef3032091c1c4f4237f663cefcab
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
