ci/macos: run with macos-13

Running with macos-latest (14) fails, because 

===> /Users/runner/work/eturnal/eturnal/c_src/fast_tls.c:21:10: fatal error: 'openssl/err.h' file not found

Difference: Intel vs. AppleM1 , so we need to understand which paths/variables to set. Afterwards we revert back to macos-latest (14).

.github/workflows/macos.yml

@@ -16,7 +16,7 @@ jobs:
 
   ci:
     name: CI | macOS with homebrew
-    runs-on: macos-latest
+    runs-on: macos-13
 
     steps:
       - name: Check out repository code