cmd/tailscale/cli: make "update" work on Windows

Updates #6995

Co-authored-by: Aaron Klotz <aaron@tailscale.com>
Change-Id: I16622f43156a70b6fbc8205239fd489d7378d57b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

scripts/check_license_headers.sh

@@ -37,6 +37,9 @@ for file in $(find $1 -name '*.go' -not -path '*/.git/*'); do
         ;;
         $1/wgengine/router/ifconfig_windows.go)
             # WireGuard copyright.
+        ;;
+        $1/cmd/tailscale/cli/authenticode_windows.go)
+            # WireGuard copyright.
         ;;
 		*_string.go)
 			# Generated file from go:generate stringer