Replace the status badge of GHA workflow in accordance with docs

[sato11]

The nightly build of the master branch has been fixed a couple of days ago (#78), yet this status badge at the top of README still indicates it's failing.

The previous version of the badge link seems not documented, while the one that's documented^1 does show it's passing. So let's just replace it.

The notation is taken from the "Create status badge" button at the dashboard^2. The ?branch=master has been removed since the badge points to the default branch by default.