[ci] Disable clad on mac-beta

Clad currently has problems with some new headers in MacOSX15.0.sdk
and crashes / errors due to __builtin_pow. Disable it for now while
a solution is worked on, so we can upgrade the nodes and keep the
build green.

(cherry picked from commit c3264ca)

.github/workflows/root-ci-config/buildconfig/mac-beta.txt

@@ -22,6 +22,7 @@ builtin_xxhash=ON
 builtin_zeromq=ON
 builtin_zstd=ON
 ccache=ON
+clad=OFF
 cocoa=ON
 fortran=OFF
 mysql=OFF