How to use GCC with Conan & CMake on MacOS - c++

Question

I am tryting to setup a build of a project on my Macbook which should use GCC with both Conan & CMake.
I have set the following environment variables:
export CC=/usr/local/bin/gcc-12
export CXX=/usr/local/bin/g++-12
When I run Conan all libraries are installed (and built if missing):
conan install ../ --build=missing
My .conan/profiles/default shows that Conan has used gcc-12.
[settings]
os=Macos
os_build=Macos
arch=armv8
arch_build=armv8
compiler=gcc
compiler.version=12
compiler.libcxx=libstdc++11
build_type=Release
[options]
[build_requires]
[env]
When I run CMake with:
cmake -DCMAKE_C_COMPILER=/usr/local/bin/gcc-12 -DCMAKE_CXX_COMPILER=/usr/local/bin/g++-12 ../
I get the following error
cmake ../
-- Ubuntu: Using Conan to manage dependencies
-- Conan: Adjusting output directories
-- Conan: Using cmake global configuration
-- Conan: Adjusting default RPATHs Conan policies
-- Conan: Adjusting language standard
-- Current conanbuildinfo.cmake directory:
~/some-dir/build
CMake Error at build/conanbuildinfo.cmake:1812 (message):
Incorrect 'apple-clang', is not the one detected by CMake: 'GNU'
Call Stack (most recent call first):
build/conanbuildinfo.cmake:1369 (conan_check_compiler)
CMakeLists.txt:62 (conan_basic_setup)
build/conanbuildinfo.cmake:1369 contains the following:
message(FATAL_ERROR "Incorrect '${CONAN_COMPILER}', is not the one
detected by CMake: '${CMAKE_CXX_COMPILER_ID}'")
This indicates that CMake has detected the Conan compiler as AppleClang NOT GCC
I cannot see where conanbuildinfo.cmake gets the value for CONAN_COMPILER from.
How can I force CMake & Conan to use GCC-12?