Use || true instead

We apparently can't use continue-on-error like this:
```
EmilienM/devstack-action/v0.2/action.yaml (Line: 29, Col: 7): Unexpected value 'continue-on-error'
```

Author: EmilienM

action.yaml

@@ -25,8 +25,7 @@ runs:
       shell: bash
     # See http://lists.openstack.org/pipermail/openstack-discuss/2020-October/018060.html
     - name: Remove python3-simplejson
-      run: sudo apt-get purge -y python3-simplejson
-      continue-on-error: true
+      run: sudo apt-get purge -y python3-simplejson || true
       shell: bash
     - name: Checkout Devstack
       uses: actions/checkout@v2