* checkVarDefs: don't check in closed terms, which don't have

  undefined variables by definition.  This matters for the
  implementation of "with", which does a call to checkVarDefs to see
  if the body of the with has no undefined variables.  (It can't be
  checked at parse time because you don't know which variables are in
  the "with" attribute set.)  If we check closed terms, then we check
  not just the with body but also the substituted terms, which are
  typically very large.  This is the cause of the poor nix-env
  performance on Nixpkgs lately.  It didn't happen earlier because
  "with" wasn't used very often in the past.

  This fix improves nix-env performance roughly 60x on current Nixpkgs.
  nix-env -qa is down from 29.3s to 0.5s on my laptop, and nix-env -qa
  --out-path is down from 229s to 3.39s.  Not bad for a 1-line fix :-)

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