Makefile: support for a different address (remote) for CometBFT repository

[cason]

The Makefile currently downloads CometBFT's repository from its full HTTP address: https://github.com/cometbft/cometbft.git.

While this default behavior is possibly necessary, it should be possible to setup a different address (remote) from which to clone the repository. For instance, a local copy of it or a GIT remote address which is less affected by rate control.

The suggestion is to create a variable on Makefile, say COMETBFT_REMOTE. When it is set, the build role should clone the repository from the configured address. When not set, it should clone it from the above mentioned default remote.