[bug] Fix metal linker error when TI_WITH_METAL=OFF

[qiao-bo]

Related issue = #4464

This PR fixes the following linker error

/usr/bin/ld: CMakeFiles/taichi_isolated_core.dir/taichi/program/program.cpp.o: in function `taichi::lang::Program::Program(taichi::Arch)':
program.cpp:(.text+0x561): undefined reference to `taichi::lang::metal::is_metal_api_available()'
/usr/bin/ld: program.cpp:(.text+0x581): undefined reference to `taichi::lang::MetalProgramImpl::MetalProgramImpl(taichi::lang::CompileConfig&)'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

To reproduce: #4464 (comment)

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[ailzhang]

Thanks!