fix: pin macOS CI to Node 20.9.0 for isolated-vm prebuilt binaries

isolated-vm@6.0.2 doesn't have prebuilt binaries for Node 20.19.6 on
macOS ARM64. When it tries to compile from source, it fails with V8 API
incompatibility errors (SourceLocation not found).

Solution: Pin macOS runners to Node 20.9.0 which has prebuilt binaries.
This ensures isolated-vm installs successfully and all security features
work as designed.

Fixes: #isolated-vm-compilation-failures

Author: Arul-

.github/workflows/ci.yml

@@ -30,6 +30,11 @@ jobs:
             node-version: 18.x
           - os: macos-latest
             node-version: 18.x
+        include:
+          # Pin macOS to Node 20.9.0 for isolated-vm prebuilt binary compatibility
+          - os: macos-latest
+            node-version: 20.9.0
+            exact-version: true
 
     steps:
       - name: Checkout code
@@ -126,7 +131,7 @@ jobs:
       - name: Setup Node.js
         uses: actions/setup-node@v4
         with:
-          node-version: '20.x'
+          node-version: '20.9.0'
           cache: 'npm'
 
       - name: Install dependencies