version: fix result from tailscale-version.sh when not using a git source (#117)

If the developer has used `go mod vendor` or `go work use ../tailscale`
then the version script was returning "tailscale.com" instead of hitting
the error case.

This now makes the outcome with the Makefile slightly worse, which needs
to be fixed up next.

Updates #cleanup

version/tailscale-version.sh

@@ -11,10 +11,10 @@ set -euo pipefail
 
 go_list=$(go list -m tailscale.com)
 # go list outputs `tailscale.com <version>`. Extract the version.
-mod_version=${go_list##* }
+mod_version=${go_list#tailscale.com}
 
 if [ -z "$mod_version" ]; then
-	echo "no version reported by go list -m tailscale.com: $go_list"
+	echo >&2 "no version reported by go list -m tailscale.com: $go_list"
 	exit 1
 fi