Use built-in GITHUB_API_URL

[erodewald]

Fix: #27

My bog-standard configuration used in a GHES instance to verify it:

    - uses: erodewald/pr-size-labeler@master
      with:
        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
        xs_max_size: '10'
        s_max_size: '100'
        m_max_size: '500'
        l_max_size: '1000'
        fail_if_xl: false