internal/core/convert: avoid repeated OpContext.Env calls
They're also only used for adt.MakeRootConjunct,
which happily uses an empty environment when none is given.
Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I8cf66bfd91eb445e50851a5fc875499d4db413c4
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1229431
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Matthew Sackman <matthew@cue.works>