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>

collector/flake_test.go

@@ -2,14 +2,14 @@ package collector
 
 import (
 	"encoding/json"
-	"os/exec"
+	"os"
 	"testing"
 )
 
 func TestGetNixosVersionInfo(t *testing.T) {
 	// Skip if nixos-version command is not available
-	if _, err := exec.LookPath("nixos-version"); err != nil {
-		t.Skip("nixos-version command not available")
+	if _, err := os.Stat(nixosVersionPath); os.IsNotExist(err) {
+		t.Skip("nixos-version command not available (not running on NixOS)")
 	}
 
 	info, err := getNixosVersionInfo()