[FIX] mail_environment: add search field for fetchmail.server's is_ssl
Fixes
```
while parsing /home/odoo/16.0/addons/mail/views/fetchmail_views.xml:88
Error while validating view near:
<separator/>
<filter string="SSL" name="ssl" domain="[('is_ssl', '=', True)]" help="If SSL required."/>
<separator/>
<filter string="Archived" name="inactive" domain="[('active', '=', False)]"/>
</search>
Unsearchable field 'is_ssl' in path 'is_ssl' in domain of <filter name="ssl"> ([('is_ssl', '=', True)]))
```
While upgrading --all on a setup with mail_environment installed.
dbc081ce9a