`CantConvert` is probably not the most appropriate, but semantically we
try to convert the mess to a record.

Same behavior as `$env.PROTECTED = ...` assignment evaluation or
`load-env`

Record is more readable and explicit although slightly more work to
generate in code than a list

`FOO=bar command` syntax gets desugared in the parser to a `with-env`
call.

Thus migrate to the recommended record form.

This was otherwise traversing the engine state for the definition of
`with-env` even if it is not needed here. May improve performance
slightly (if llvm did not come to the conclusion that this had no
side-effects and could reorder)