Ignore OPAMSWITCH when displaying environment varilables (eg. on 'opam config env' or 'opam switch foo').
Ignore OPAMSWITCH when displaying environment varilables (eg. on 'opam config env' or 'opam switch foo').

This fixes a regression introduced by 27b2f81deb0213e12e1aa1cf833b4d991a8653f6 where you cannot switch anymore when you have OPAMSWITCH set.

abate/opam,abate/opam