action: upgrade pip during local test

Upgrade pip to latest during local test.

.github/workflows/generate_release_oldwin.yml

@@ -25,7 +25,8 @@ jobs:
           python -m venv .venv
           Set-ExecutionPolicy -ExecutionPolicy Unrestricted -Scope CurrentUser
           .\.venv\Scripts\Activate.ps1
-          .\.venv\Scripts\pip.exe install -r requirements.txt
+          python -m pip install --upgrade pip
+          python -m pip install -r requirements.txt
           python -c "import server; from youtube import util; url = 'https://cloudflare.com/cdn-cgi/trace'; resp = util.fetch_url(url, report_text='Getting CF trace report'); print(resp.decode())" && deactivate
       - name: Generating release
         id: generating-release