fix!: `env.sh` sourceable should not be in `$PATH`

Created by: c-dilks

Having env.sh in one's $PATH means that when some shells run source env.sh from anywhere, it may prioritize the env.sh from the coatjava installation, rather than the env.sh in the caller's present working directory. Since both coatjava's env.sh and the caller's ./env.sh may be silent, this can lead to confusion as to why the local ./env.sh is not working.

This PR moves env.sh to libexec/, which should not be in one's $PATH. Any downstream code which calls source env.sh may be affected, which is why the conventional commit label is fix! rather than fix (i.e., potential breaking change).