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.
Merge branch 'master' of github.com:OCamlPro/opam