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-cert/main.go

@@ -5,10 +5,28 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"runtime/debug"
+	"strings"
 )
 
+// A version string that can be set with
+//
+//	-ldflags "-X main.Build=SOMEVERSION"
+//
+// at compile-time.
 var Build string
 
+func init() {
+	if Build == "" {
+		info, ok := debug.ReadBuildInfo()
+		if !ok {
+			return
+		}
+
+		Build = strings.TrimPrefix(info.Main.Version, "v")
+	}
+}
+
 type helpError struct {
 	s string
 }