run wrapped-due-to-env commands on unix via the env program

First, check if the env program exists. If it does, it is faster than
doing it via a python script `basically-env.py` that maybe imports all
of mesonbuild.* as a side effect of project structure.

We do not, however, use env for setting up PATH additions, since env can
override an environment variable but not extend it. So in that case we
still need to wrap the command via python.

By default, all run_targets (at least) are wrapped and now wrap via the
`env` program as they export e.g.
MESONINTROSPECT='/usr/bin/meson introspect'

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