Alembic: avoid red overwrite warning when opening a file

Pass `FILE_OPENFILE` instead of `FILE_SAVE` when selecting a file for
reading.

source/blender/editors/io/io_cache.c

@@ -130,7 +130,7 @@ void CACHEFILE_OT_open(wmOperatorType *ot)
   WM_operator_properties_filesel(ot,
                                  FILE_TYPE_ALEMBIC | FILE_TYPE_FOLDER,
                                  FILE_BLENDER,
-                                 FILE_SAVE,
+                                 FILE_OPENFILE,
                                  WM_FILESEL_FILEPATH,
                                  FILE_DEFAULTDISPLAY,
                                  FILE_SORT_DEFAULT);