populate default Build version if missing (#1386)

* populate default Build version if missing

Use the Go module information built into the binary if the Build var
wasn't set during the build.

This means if you install via a specific tag, you get:

    go install github.com/slackhq/nebula/cmd/nebula@v1.9.5

    $ nebula -version
    Version: 1.9.5

And if you install master, you get:

    go install github.com/slackhq/nebula/cmd/nebula@master

    $ nebula -version
    Version: 1.9.5-0.20250408154034-18279ed17b10

* also default in the library

* cleanup

cmd/nebula-service/main.go

@@ -4,6 +4,8 @@ import (
 	"flag"
 	"fmt"
 	"os"
+	"runtime/debug"
+	"strings"
 
 	"github.com/sirupsen/logrus"
 	"github.com/slackhq/nebula"
@@ -18,6 +20,17 @@ import (
 // at compile-time.
 var Build string
 
+func init() {
+	if Build == "" {
+		info, ok := debug.ReadBuildInfo()
+		if !ok {
+			return
+		}
+
+		Build = strings.TrimPrefix(info.Main.Version, "v")
+	}
+}
+
 func main() {
 	serviceFlag := flag.String("service", "", "Control the system service.")
 	configPath := flag.String("config", "", "Path to either a file or directory to load configuration from")