implements isolated install sanity-checking for Cabal installs

lib/GHCup.hs

@@ -405,6 +405,7 @@ installCabalBindist :: ( MonadMask m
 #if !defined(TAR)
                           , ArchiveResult
 #endif
+                          , IsolatedDirNotEmpty
                           ]
                          m
                          ()
@@ -425,7 +426,8 @@ installCabalBindist dlinfo ver isoFilepath = do
         )
         (throwE $ AlreadyInstalled Cabal ver)
 
-    _ -> pure () -- check isn't required in isolated installs
+    Just isoDir ->
+      liftE $ isolatedInstallSanityCheck isoDir
 
   -- download (or use cached version)
   dl <- liftE $ downloadCached dlinfo Nothing
@@ -498,6 +500,7 @@ installCabalBin :: ( MonadMask m
 #if !defined(TAR)
                       , ArchiveResult
 #endif
+                      , IsolatedDirNotEmpty
                       ]
                      m
                      ()