License check doesn't conform to REUSE standard

Verilator just tripped on this, it is REUSE compliant https://reuse.software/ which means it has a LICENSES directory. But the license action here doesn't pass with a "LICENSES" directory, it instead gives:

From e.g. https://github.com/chipsalliance/sv-tests/actions/runs/21532313805/job/62050768965?pr=8118

```
Error: third_party/tools/verilator: A license file was not found (tried ['LICENSE', 'LICENCE', 'COPYING', 'COPYRIGHT'])
```

Please change the action to pass if LICENSES (as a directory not a file) exists, thanks.