Replace shell based linux sandbox setup with go code

cmd/fence/linux_bootstrap_stub.go

@@ -0,0 +1,15 @@
+//go:build !linux
+
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+// runLinuxBootstrapWrapper is a stub for non-Linux platforms.
+// The bootstrap wrapper is only needed inside Linux sandboxes.
+func runLinuxBootstrapWrapper() {
+	fmt.Fprintln(os.Stderr, "[fence] --linux-bootstrap is only supported on Linux")
+	os.Exit(1)
+}