fix(CI): Call python to upgrade pip on Windows (XRPLF#4768)

In Windows, we need to call `python` in order for the `pip` upgrade
command to work.

This changes the GitHub Actions Windows CI job to use the correct
command to upgrade PIP, fixing this error:

```
ERROR: To modify pip, please run the following command:
C:\hostedtoolcache\windows\Python\3.9.13\x64\python.exe -m pip install --upgrade pip
```

A future task is to make job run on heavy Windows runners so that it
doesn't take so long.

Context: XRPLF#4596

.github/workflows/windows.yml

@@ -36,7 +36,7 @@ jobs:
         id: pip-cache
         shell: bash
         run: |
-          pip install --upgrade pip
+          python -m pip install --upgrade pip
           echo "dir=$(pip cache dir)" | tee ${GITHUB_OUTPUT}
       - name: restore Python cache directory
         uses: actions/cache@v3