The default behavior of subprocess is to pull executables from a
hardcoded list, which doesn't work when opam is installed manually in
the user's home directory.
That avoids having multiple distinct opam root directories per opam lister instance. The
current opam commands used by the lister are actually listing specifically per instance.
Related to P1171
We should avoid side-effects in the constructor as much as possible. That avoids
surprising behavior at object instantiation time. The state if needed must be
initialized into the `swh.lister.pattern.Lister.get_pages` method, as preconized in the
class docstring.
This also fixes the current test that actually bootstrap a real opam local "clone" in
/tmp.
Related to T3590
