#7339 changed the root directory logic to find the ancestor of the cwd
with a go.mod file. This works when running the the binary from this
repo directly, but breaks when we're a dependency in another repo.
Allow the directory to be passed in via a -rootdir flag (the repo that
depends on it can then use `go list -m -f '{{.Dir}}' tailscale.com`
or similar to pass in the value).
Updates tailscale/corp#10165
Signed-off-by: Mihai Parparita <mihai@tailscale.com>
02582083d5