ci: switch appveyor to gce builders to try debugging the 259 failure

appveyor.yml

@@ -5,6 +5,11 @@ environment:
   # server goes down presumably. See #43333 for more info
   CARGO_HTTP_CHECK_REVOKE: false
 
+  # Execute the builds on GCE instead of Hyper-V. Those builders have a 3-4
+  # minute startup overhead, but AppVeyor support recommended this as a
+  # possible solution for #58160 (spurious 259 exit codes)
+  appveyor_build_worker_cloud: gce
+
   matrix:
   # 32/64 bit MSVC tests
   - MSYS_BITS: 64