CLI admin diagnostics: only check readability of var folder

In a multi-user environment where group members have permissions to read but not write the var/ folder of the server, bin/omero admin diagnostics currently fails with a "Cannot access" exception. This commit relaxes the check_access() condition of the diagnostics subcommand to check the folder (and subfolders) readability only.

To test this PR, ssh to `trout.openmicroscopy.org` and check `bin/omero admin diagnostics` does not fail on the deployed server.

/cc @aleksandra-tarkowska