chore: bump nightly version to 0.18.0-nightly-20250915 (#2117)

Author: github-actions[bot]

variables/variables-nightly.ts

@@ -1,5 +1,5 @@
 export const variables = {
-  greptimedbVersion: 'v0.17.0-nightly-20250901',
+  greptimedbVersion: 'v0.18.0-nightly-20250915',
   prometheusVersion: 'v2.52.0',
   nodeExporterVersion: 'v1.8.0',
   goSdkVersion: 'v0.6.2',