[ci] Disable builtin_vc on mac15

It fails to build with the latest SDK because of missing includes and
was already off for mac26 and mac-beta (probably for that reason).

Author: hahnjo

.github/workflows/root-ci-config/buildconfig/mac15.txt

@@ -16,7 +16,6 @@ builtin_openssl=ON
 builtin_pcre=ON
 builtin_tbb=ON
 builtin_unuran=ON
-builtin_vc=ON
 builtin_vdt=ON
 builtin_veccore=ON
 builtin_xrootd=ON