cmake: don't prefer homebrew clang on the CI

cmake/LLVM.cmake

@@ -1,6 +1,10 @@
 if(NOT CMAKE_TOOLCHAIN_FILE)
 # Prefer the homebrew version because xcode clang doesn't have detect_leaks
 # Note: CMAKE_SYSTEM_NAME is not available yet.
+#
+# Don't prefer homebrew clang on the CI because we want
+# to produce universal binaries there.
+if(NOT DEFINED ENV{CI})
 if(NOT DEFINED CUSTOM_LLVM_HOME)
 # Note: The recent macOS SDKs (eg. math.h from macOS SDK 15) assume that
 # _Float16 is available. In case of x86, it's only available for the
@@ -9,6 +13,7 @@ if(NOT DEFINED CUSTOM_LLVM_HOME)
 set(CUSTOM_LLVM_HOME /usr/local/opt/llvm@15)
 endif()
 endif()
+endif()
 
 if(DEFINED CUSTOM_LLVM_HOME)
 find_program(CUSTOM_CLANG ${CUSTOM_LLVM_HOME}/bin/clang)