use last stable xcode 12 for mac builds

Certain terraform-exec tests run versions of Terraform earlier than 0.12 in order to test compatibility behaviour.

Any version of Terraform complied with Go < 1.11 causes errors when run on macOS Monterey 12 or later, which corresponds to Xcode version 13 or later. Please see the following pages for more information:
https://github.com/golang/go/wiki/MacOS12BSDThreadRegisterIssue
https://xcodereleases.com/

We must therefore use Xcode 12 for these tests. CircleCI supports only the latest stable version, currently v12.5.1.

When Xcode 16 is released, CircleCI will drop support for Xcode 12, so we will need to resolve this problem another way.

.circleci/config.yml

@@ -37,7 +37,7 @@ jobs:
   # combined due to slowness of Go install
   macosbuildtest:
     macos:
-      xcode: "12.0.0"
+      xcode: "12.5.1"
     steps:
       - macos_install_go
       - checkout