fix: Write temporary files for export to the t directory. (#7975)

Write to t instead of /tmp which in Cloud is mapped to the node storage instead
of the attached volume. /tmp can fill up easily since it's typically smaller than the
allocated storage for the Dgraph Alpha disk where the t directory is.

worker/export.go

@@ -473,7 +473,7 @@ func (l *localExportStorage) finishWriting(fs ...*fileWriter) (ExportedFiles, er
 }
 
 func newRemoteExportStorage(in *pb.ExportRequest, backupName string) (*remoteExportStorage, error) {
-	tmpDir, err := ioutil.TempDir("", "export")
+	tmpDir, err := ioutil.TempDir(x.WorkerConfig.TmpDir, "export")
 	if err != nil {
 		return nil, err
 	}