Adds isolate install functionality to 'Cabal' tool installs

app/ghcup/Main.hs

@@ -1679,7 +1679,15 @@ Report bugs at <https://gitlab.haskell.org/haskell/ghcup-hs/issues>|]
 
 
           let installCabal InstallOptions{..} =
-                (case instBindist of
+                (case isolateDir of
+                  Just isoDir ->
+                    runInstTool instPlatform $ do
+                    (v, vi) <- liftE $ fromVersion instVer Cabal
+                    let cabalVersion = (_tvVersion v)
+                    liftE $ installCabalBinIsolated isoDir cabalVersion
+                    pure vi
+                  Nothing ->
+                    case instBindist of
                        Nothing -> runInstTool instPlatform $ do
                          (v, vi) <- liftE $ fromVersion instVer Cabal
                          liftE $ installCabalBin (_tvVersion v)

lib/GHCup.hs

@@ -487,6 +487,57 @@ installCabal' path inst ver = do
     destPath
   lift $ chmod_755 destPath
 
+-- | Installs GHC to a specified location, doesn't make any symlinks.
+installCabalBinIsolated :: ( MonadMask m
+                           , MonadCatch m
+                           , MonadReader env m
+                           , HasPlatformReq env
+                           , HasGHCupInfo env
+                           , HasDirs env
+                           , HasSettings env
+                           , MonadLogger m
+                           , MonadResource m
+                           , MonadIO m
+                           , MonadUnliftIO m
+                           , MonadFail m
+                           )
+                        => FilePath
+                        -> Version
+                        -> Excepts
+                             '[ AlreadyInstalled
+                              , CopyError
+                              , DigestError
+                              , DownloadFailed
+                              , NoDownload
+                              , NotInstalled
+                              , UnknownArchive
+                              , TarDirDoesNotExist
+#if !defined(TAR)
+                              , ArchiveResult
+#endif
+                              ]
+                             m
+                             ()
+installCabalBinIsolated isoDir ver = do
+  dlinfo <- liftE $ getDownloadInfo Cabal ver
+  lift $ $(logDebug) [i|Requested to install cabal version #{ver}|]
+
+  -- download (or use cached version)
+  dl <- liftE $ downloadCached dlinfo Nothing
+
+  PlatformRequest {_rPlatform} <- lift getPlatformReq
+
+  -- unpack
+  tmpUnpack <- lift withGHCupTmpDir
+  liftE $ unpackToDir tmpUnpack dl
+  void $ lift $ darwinNotarization _rPlatform tmpUnpack
+
+  -- the subdir of the archive where we do the work
+  workdir <- maybe (pure tmpUnpack) (liftE . intoSubdir tmpUnpack) (view dlSubdir dlinfo)
+
+  lift $ $(logInfo) [i|isolated installing Cabal to #{isoDir}|]
+  liftE $ installCabal' workdir isoDir ver
+
 
 -- | Installs cabal into @~\/.ghcup\/bin/cabal-\<ver\>@ and
 -- creates a default @cabal -> cabal-x.y.z.q@ symlink for