fix pypi upload: skip-existing -> skip_existing

aaargh... https://github.com/google/brotli-wheels/actions/runs/11402512035/job/31728799137#step:4:1
```
Unexpected input(s) 'skip-existing', valid inputs are [... 'skip_existing', ...]
```

.github/workflows/ci.yml

@@ -157,4 +157,4 @@ jobs:
       with:
         user: __token__
         password: ${{ secrets.PYPI_PASSWORD }}
-        skip-existing: true
+        skip_existing: true