Fix work-in-progress check

Sorry, I've bodged it when adding the configuration. Currently, if the `CHECK_WIP` flag is set to `False` the checks are gonna be run even if it's a _work-in-progress_ PR. If the `CHECK_WIP` flag is set to `True` the checks are always done, otherwise it must verifies that it's indeed a _work-in-progress_ one.

### Before

| CHECK_WIP | is_wip | check |
| ----------------- | ------- | --------| 
| True | True | True |
| True | False | **False** |
| False | True | **True** |
| False | False | True |

### After

| CHECK_WIP | is_wip | check |
| ----------------- | ------- | --------| 
| True | True | True |
| True | False | **True** |
| False | True | **False** |
| False | False | True |