randomly select gcp zone

Author: f-gate

.github/workflows/build.yml

@@ -14,7 +14,8 @@ on:
       - "**.md"
 env:
   CARGO_TERM_COLOR: always
-  GCP_ZONE: europe-west3
+  GCP_ZONES: (europe-central2-a, europe-central2-b, europe-central2-c, europe-north1-a, europe-north1-b, europe-north1-c, europe-west1-b, europe-west1-c)
+  GCP_ZONE: ${GCP_ZONES[$((RANDOM % 8)]})
 
 jobs:
   check_branch: