When launching setup in another dir we must use an adjusted path

The path to the binary we're running must be accessable from
that other directory. The fix is to use currentDir </> setup

cabal-install/Distribution/Client/SetupWrapper.hs

@@ -54,7 +54,7 @@ import Distribution.Text
 import Distribution.Verbosity
          ( Verbosity )
 
-import System.Directory  ( doesFileExist, getModificationTime )
+import System.Directory  ( doesFileExist, getModificationTime, getCurrentDirectory )
 import System.FilePath   ( (</>), (<.>) )
 import System.IO.Error   ( isDoesNotExistError )
 import System.IO         ( Handle )
@@ -281,7 +281,8 @@ externalSetupMethod verbosity options pkg bt mkargs = do
     Just logHandle -> do
       info verbosity $ unwords (setupProgFile : args)
       info verbosity $ "Redirecting build log to " ++ show logHandle
-      process <- runProcess setupProgFile args
+      currentDir <- getCurrentDirectory
+      process <- runProcess (currentDir </> setupProgFile) args
                    (useWorkingDir options) Nothing
                    Nothing (Just logHandle) (Just logHandle)
       exitCode <- waitForProcess process