Fix mac builds ... once again

Apparently, xcode 14 has a reported linker bug which prevents a
successful build. The recommended workaround is to revert toolchain to
last known good version (v13.4.1). More information can be found here -

actions/runner-images#6350

.github/workflows/main.yml

@@ -599,6 +599,11 @@ jobs:
 
     - name: Configure compiler
       run: |
+        # Default xcode version 14.0.1 has reported bugs with linker
+        # Current recommended workaround is to downgrade to last known good version.
+        # https://github.com/actions/runner-images/issues/6350
+        sudo xcode-select -s '/Applications/Xcode_13.4.1.app/Contents/Developer'
+
         if [ "${{matrix.compiler}}" == "clang" ]; then
           echo 'CC=clang' >> $GITHUB_ENV
           echo 'CXX=clang++' >> $GITHUB_ENV