Reports whether the current binary was built with Tailscale's
custom Go toolchain (the "tailscale_go" build tag).
For https://github.com/tailscale/go/pull/165
Updates tailscale/corp#39430
Change-Id: Ica437582ddf55d7df48b1453bad03ce14b1c0949
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>