Use non-associative for ‘or’ selector

When an expression looks like this:

  a.b or (0 == 0)

The precedence wasn’t being applied correctly in the pretty printer.
The parenthesis were left off so that the pretty printer produced,

  a.b or 0 == 0

While they look similar, without the parenthesis the expression would
take (a.b or 0) as one expression. This caused an error in the
Pretty/Parse Property tests. Disabling associativity for the (0 == 0)
part fixed this.

Fixes #305

src/Nix/Pretty.hs

@@ -205,7 +205,8 @@ exprFNixDoc = \case
     NSelect r attr o ->
       (if isJust o then leastPrecedence else flip mkNixDoc selectOp) $
           wrapPath selectOp r <> dot <> prettySelector attr <> ordoc
-      where ordoc = maybe empty (((space <> text "or") <+>) . wrapParens selectOp) o
+      where
+        ordoc = maybe empty (((space <> text "or") <+>) . wrapParens appOpNonAssoc) o
     NHasAttr r attr ->
         mkNixDoc (wrapParens hasAttrOp r <+> text "?" <+> prettySelector attr) hasAttrOp
     NEnvPath p -> simpleExpr $ text ("<" ++ p ++ ">")