* Small env fixes: for git submodules & python3
Using `/usr/bin/env python3` is preferred over `/usr/bin/python3`
because it is compatible with virtualenvs, etc. It picks the version of
`python3` currently on the `PATH`, rather than the system-wide default.
For the git submodules, I had two issues:
The ZAPD repo is currently pointing at a commit not on the master
branch, which has to be specifically fetched. Adding `--remote` to the
update command fixes this.
The decomp-permuter has primary branch `main` not `master` like this
repo, so this has to be specified in `.gitmodules` when using `--remote`.
* Rework `git submodule update...` part of Makefile
kyleburnette