pkg: Add env vars interface for enabling and disabling package management during single command executions

[shonfeder]

#12819 called for exposing this configuration via environment variables, along the lines of

DUNE_PKG=1 dune build # enabled
DUNE_PKG=0 dune build # disabled
DUNE_PKG=  dune build # will run with the default behavior given the file systems

But this requirement wasn't addressed by #13458

This will make it easy for users to opt-in to using dune pkg for terminal sessions (or on a per-repo basis, e.g. via direnv), without requiring dirty config files or awkward workarounds. This can also be used in CI.