Correctly un-export an env var when it is shadowed

Prior to this fix, if you exported a variable in one scope
and then unexported it in the next, it would remain exported.
Example:

    set -gx VAR 1
    function foo; set -l VAR; env; end
    foo

Here 'VAR' would be exported to 'env' because we failed to
notice that the env var is shadowed by an unexported variable.
This occurred at env var computation time, not in env_set!

Fixes #2132

tests/test3.out

@@ -16,6 +16,8 @@ Test 15 pass
 Foo change detected
 Foo change detected
 Test 16 pass
+__fish_test_env17=UNSHADOWED
+SHADOWED
 Testing Universal Startup
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