certspotterVersion: also allow -X=main.Version

debug.ReadBuildInfo() doesn't work for the Debian package, where there
is no git repository (at best; could be the Debian packaging one). The
go module doesn't have a version either.

Add a quick shortcut at the top, that sets the version to main.Version
if it's non-empty -- which is by default, so no change in behavior.

Effectively, this allows one to use the standard
-ldflags=-X=main.Version to set the version, like we intend to do in the
Debian package.

cmd/certspotter/main.go

@@ -30,10 +30,14 @@ import (
 )
 
 var programName = os.Args[0]
+var Version = ""
 
 const defaultLogList = "https://loglist.certspotter.org/monitor.json"
 
 func certspotterVersion() string {
+	if Version != "" {
+		return Version
+	}
 	info, ok := debug.ReadBuildInfo()
 	if !ok {
 		return "unknown"