Avoid clobbering last error with NO_ERROR when GetEnvironmentVariableA
succeeds, matching the behavior of GetEnvironmentVariableW and
Windows.

Instead of naively saving and restoring the last error, call
RtlQueryEnvironmentVariable_U directly to avoid unnecessarily setting
it in the first place.
Signed-off-by: Vladimir Panteleev <git@vladimir.panteleev.md>
Signed-off-by: Gijs Vermeulen <gijsvrm@gmail.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>