Line-buffer stdout and stderr

I though this was the default, but apparently not! This seems to fix
output ordering issues.

Noticed here: #10573 (comment)

cabal-validate/src/Main.hs

@@ -15,6 +15,7 @@ import qualified Data.Text.Lazy.Encoding as T (decodeUtf8)
 import Data.Version (makeVersion, showVersion)
 import System.FilePath ((</>))
 import System.Process.Typed (proc, readProcessStdout_)
+import System.IO (hSetBuffering, BufferMode(LineBuffering), stdout, stderr)
 
 import Cli (Compiler (..), HackageTests (..), Opts (..), parseOpts, whenVerbose)
 import OutputUtil (printHeader, withTiming)
@@ -24,6 +25,18 @@ import Step (Step (..), displayStep)
 -- | Entry-point for @cabal-validate@.
 main :: IO ()
 main = do
+  -- You'd _think_ that line-buffering for stdout and stderr would be the
+  -- default behavior, and the documentation makes gestures at it, but it
+  -- appears to not be the case!
+  --
+  -- > For most implementations, physical files will normally be
+  -- > block-buffered and terminals will normally be line-buffered.
+  --
+  -- However, on GitHub Actions and on my machine (macOS M1), adding these
+  -- lines makes output appear in the correct order!
+  hSetBuffering stdout LineBuffering
+  hSetBuffering stderr LineBuffering
+
   opts <- parseOpts
   printConfig opts
   printToolVersions opts