Fix T86063: support 'Relative Path' setting opening (alembic) caches

This was reported as opening alembic caches ignoring the
'use_relative_paths' preference, but this operator just did not have
this setting. Fortunately, adding this is just a simple switch.

Maniphest Tasks: T86063

Differential Revision: https://developer.blender.org/D10568

source/blender/editors/io/io_cache.c

@@ -131,7 +131,7 @@ void CACHEFILE_OT_open(wmOperatorType *ot)
                                  FILE_TYPE_ALEMBIC | FILE_TYPE_FOLDER,
                                  FILE_BLENDER,
                                  FILE_OPENFILE,
-                                 WM_FILESEL_FILEPATH,
+                                 WM_FILESEL_FILEPATH | WM_FILESEL_RELPATH,
                                  FILE_DEFAULTDISPLAY,
                                  FILE_SORT_DEFAULT);
 }