fix: use absolute path for nixos-version executable

Use /run/current-system/sw/bin/nixos-version instead of relying on
PATH, since the systemd service may not have the system binaries in
its PATH.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

main.go

@@ -15,7 +15,7 @@ import (
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
-const version = "0.2.2"
+const version = "0.2.3"
 
 func main() {
 	cfg, err := config.Parse()