CI: pin the MAC OS version to macos-11

The Rust Client SDK cannot be compiled in macOS-12.
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 1aebf5f..e2fe40e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -23,7 +23,7 @@
 
 jobs:
   teaclave-client-sdk-macos:
-    runs-on: macos-latest
+    runs-on: macos-11
     steps:
       - uses: actions/checkout@v3
         with: