adds rudimentary isolate capability to ghcup install ghc command

app/ghcup/Main.hs

@@ -1631,7 +1631,14 @@ Report bugs at <https://gitlab.haskell.org/haskell/ghcup-hs/issues>|]
           -----------------------
 
           let installGHC InstallOptions{..} =
-                  (case instBindist of
+                (case isolateDir of
+                   Just isoDir -> runInstTool instPlatform $ do
+                    (v, vi) <- liftE $ fromVersion instVer GHC
+                    let ghcVersion = _tvVersion v
+                    liftE $ installGHCBinIsolated isoDir ghcVersion
+                    pure vi
+                   Nothing ->
+                     case instBindist of
                        Nothing -> runInstTool instPlatform $ do
                          (v, vi) <- liftE $ fromVersion instVer GHC
                          liftE $ installGHCBin (_tvVersion v)

lib/GHCup.hs

@@ -229,6 +229,49 @@ installGHCBindist dlinfo ver = do
         lift $ $(logWarn) "environments). If you encounter problems, unset CC and LD and reinstall."
 
 
+-- | Installs GHC to a specified location, doesn't make any symlinks.
+installGHCBinIsolated :: ( MonadFail m
+                 , MonadMask m
+                 , MonadCatch m
+                 , MonadReader env m
+                 , HasPlatformReq env
+                 , HasGHCupInfo env
+                 , HasDirs env
+                 , HasSettings env
+                 , MonadLogger m
+                 , MonadResource m
+                 , MonadIO m
+                 , MonadUnliftIO m
+                 )
+              => FilePath
+              -> Version         -- ^ the version to install
+              -> Excepts
+                   '[ AlreadyInstalled
+                    , BuildFailed
+                    , DigestError
+                    , DownloadFailed
+                    , NoDownload
+                    , NotInstalled
+                    , UnknownArchive
+                    , TarDirDoesNotExist
+#if !defined(TAR)
+                    , ArchiveResult
+#endif
+                    ]
+                   m
+                   ()
+
+installGHCBinIsolated isoDir ver = do
+  dlinfo <- liftE $ getDownloadInfo GHC ver
+
+  lift $ $(logDebug) [i|Requested to install GHC with #{ver}|]
+
+  -- download (or use cached version)
+  dl <- liftE $ downloadCached dlinfo Nothing
+
+  liftE $ installPackedGHC dl (view dlSubdir dlinfo) isoDir ver
+
+
 -- | Install a packed GHC distribution. This only deals with unpacking and the GHC
 -- build system and nothing else.
 installPackedGHC :: ( MonadMask m