Ensure that later default file values for variable replace earlier ones.

The semigroup instance for doctemplates Context does a left-biased
union on the underlying Map.  That means that if you union
`{a: 1}` and `{a: 2}`, you get `{a: 1}`.  This commit causes
pandoc to do the operation in the opposite order, so that later
default files take precedence in the values they assign to
keys.

See #5988.

src/Text/Pandoc/App/Opt.hs

@@ -180,7 +180,9 @@ doOpt (k',v) = do
       parseYAML v >>= \x -> return (\o -> o{ optTemplate = unpack <$> x })
     "variables" ->
       parseYAML v >>= \x -> return (\o -> o{ optVariables =
-                                               optVariables o <> x })
+                                               x <> optVariables o })
+      -- Note: x comes first because <> for Context is left-biased union
+      -- and we want to favor later default files. See #5988.
     "metadata" ->
       parseYAML v >>= \x -> return (\o -> o{ optMetadata = optMetadata o <>
                                                contextToMeta x })