Fix CI failures by explicitly using Intel Mac

The image used by GitHub Actions for macos-latest has started using the M1 chip, causing CI failures. By explicitly specifying macos-13, we can prevent CI from failing.

ref: [GitHub Actions: macOS 14 (Sonoma) is now available - The GitHub Blog](https://github.blog/changelog/2024-01-30-github-actions-macos-14-sonoma-is-now-available/)

.github/workflows/ci.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
   tests-on-x86-mac:
-    runs-on: macos-latest
+    runs-on: macos-13
     strategy:
       matrix:
         ruby-version: ['2.3', '2.4', '2.5', '2.6', '2.7', '3.0', '3.1', '3.2']