test(default): use `grains.get` for `oscodename` (for FreeBSD) [skip ci]

Otherwise encounter this failure in the CI on FreeBSD:

```
salt.exceptions.SaltRenderError: Jinja variable 'dict object' has no attribute 'oscodename'
[CRITICAL] Pillar render error: Rendering SLS 'openssh' failed. Please see master log for details.
```

test/salt/pillar/default.sls

   {%- endif %}
   {#- Need this on various platforms to avoid the `kitchen verify` failure as mentioned above; see: #}
   {#- * https://gitlab.com/saltstack-formulas/infrastructure/salt-image-builder/-/commit/cb6781a2bba9 #}
-  {%- if grains.os in ["Arch", "OpenBSD"] or grains.oscodename in ["openSUSE Tumbleweed"] %}
+  {%- if grains.os in ["Arch", "OpenBSD"] or grains.get("oscodename", "") in ["openSUSE Tumbleweed"] %}
   PubkeyAcceptedAlgorithms: "+ssh-rsa"
   {%- endif %}