Don't update ghcup if already latest version

Fixes #2

lib/GHCup/Errors.hs

@@ -30,6 +30,10 @@ data NoCompatiblePlatform = NoCompatiblePlatform String -- the platform we got
 data NoDownload = NoDownload
   deriving Show
 
+-- | No update available or necessary.
+data NoUpdate = NoUpdate
+  deriving Show
+
 -- | The Architecture is unknown and unsupported.
 data NoCompatibleArch = NoCompatibleArch String
   deriving Show

lib/GHCup/Utils/Prelude.hs

@@ -240,3 +240,7 @@ addToCurrentEnv :: MonadIO m
 addToCurrentEnv adds = do
   cEnv <- liftIO $ getEnvironment
   pure (adds ++ cEnv)
+
+
+pvpToVersion :: PVP -> Version
+pvpToVersion = either (\_ -> error "Couldn't convert PVP to Version") id . version . prettyPVP