use --tmp directory for building indexes (#7289)

Makes reindexing use directory passed through --tmp flag of alpha.

(cherry picked from commit 1707ecf)

posting/index.go

@@ -562,8 +562,7 @@ func (r *rebuilder) Run(ctx context.Context) error {
 
 	// We write the index in a temporary badger first and then,
 	// merge entries before writing them to p directory.
-	// TODO(Aman): If users are not happy, we could add a flag to choose this dir.
-	tmpIndexDir, err := ioutil.TempDir("", "dgraph_index_")
+	tmpIndexDir, err := ioutil.TempDir(x.WorkerConfig.TmpDir, "dgraph_index_")
 	if err != nil {
 		return errors.Wrap(err, "error creating temp dir for reindexing")
 	}