implements isolated install sanity-checking for Cabal installs

app/ghcup/BrickMain.hs

@@ -442,6 +442,7 @@ install' _ (_, ListResult {..}) = do
               , DownloadFailed
               , NoUpdate
               , TarDirDoesNotExist
+              , IsolatedDirNotEmpty
               ]
 
   run (do

app/ghcup/Main.hs

@@ -1478,6 +1478,7 @@ Report bugs at <https://gitlab.haskell.org/haskell/ghcup-hs/issues>|]
                       , TarDirDoesNotExist
                       , NextVerNotFound
                       , NoToolVersionSet
+                      , IsolatedDirNotEmpty
                       ]
 
           let runInstTool mInstPlatform action' = do

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
                      ()