build requires all submodules

[GammaSQ]

make build fails with:

> [12/18] RUN ./third_party/arduino_cli_install.sh $(readlink -f ./arduino-cli):
0.240   adding: ESPTinyUSB/ (stored 0%)
0.987 --git-url and --zip-path flags allow installing untrusted files, use it at your own risk.
0.995 Error installing Zip Library: Library install failed: library not valid
1.024   adding: arduino_base64/ (stored 0%)
1.250 --git-url and --zip-path flags allow installing untrusted files, use it at your own risk.
1.252 Error installing Zip Library: Library install failed: library not valid

because the third-party submodules are empty, as the makefile does not update them:

submodules:
	git submodule init
	# git submodule update

(I fixed this by running git submodule update --init --recursive, but I'm not a git-expert)