build: use `CMAKE_COMMAND` instead of hardcoded cmake
build: use `CMAKE_COMMAND` instead of hardcoded cmake

This ensures that the cmake used to build is the cmake invoked when
running the custom command.  In the case that `cmake` is not in PATH,
you would fail to invoke the command, failing the build.

Signed-off-by: Kim Topley <b240e9af83531ac049af314c640f722869aeb57d@apple.com>

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch