Fix cabalSet for pre-release versions

lib/GHCup/Utils.hs

@@ -259,18 +259,33 @@ cabalInstalled ver = do
 cabalSet :: (MonadIO m, MonadThrow m) => m (Maybe Version)
 cabalSet = do
   cabalbin <- (</> [rel|cabal|]) <$> liftIO ghcupBinDir
+  b        <- fmap (== SymbolicLink) $ liftIO $ getFileType cabalbin
+  if
+    | b -> do
+      liftIO $ handleIO' NoSuchThing (\_ -> pure $ Nothing) $ do
+        link <- readSymbolicLink $ toFilePath cabalbin
+        Just <$> linkVersion link
+    | otherwise -> do -- legacy behavior
       mc <- liftIO $ handleIO (\_ -> pure Nothing) $ fmap Just $ executeOut
         cabalbin
         ["--numeric-version"]
         Nothing
       fmap join $ forM mc $ \c -> if
-             | not (B.null (_stdOut c))
+        | not (B.null (_stdOut c)), _exitCode c == ExitSuccess -> do
-             , _exitCode c == ExitSuccess -> do
           let reportedVer = fst . B.spanEnd (== _lf) . _stdOut $ c
           case version $ decUTF8Safe reportedVer of
             Left  e -> throwM e
             Right r -> pure $ Just r
         | otherwise -> pure Nothing
+ where
+  linkVersion :: MonadThrow m => ByteString -> m Version
+  linkVersion bs = do
+    t <- throwEither $ E.decodeUtf8' bs
+    throwEither $ MP.parse parser "" t
+   where
+    parser =
+      MP.chunk "cabal-" *> version'
+