[bug] Remove unnecessary lower() in AotModuleBuilder::add

[PGZXB]

Related issue = #4401
Run python run_tests.py --cpp --with-offline-cache failed, reported by https://github.com/taichi-dev/taichi/actions/runs/3055000542. Reason: Generate AST key ([1]) after lowering ([2]).

[1]

taichi/taichi/codegen/codegen.cpp

Line 92 in 72804ae

std::string kernel_key = get_hashed_offline_cache_key(&config, kernel);

[2]

taichi/taichi/aot/module_builder.cpp

Lines 8 to 10 in 72804ae

	if (!kernel->lowered() && Kernel::supports_lowering(kernel->arch)) {
	kernel->lower(/*to_executable=*/!arch_uses_llvm(kernel->arch));
	}

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[ailzhang]

Nice catch!