[ci] simplify release workflow.

[sayakpaul]

What does this PR do?

Simplify release workflow like https://github.com/huggingface/transformers/blob/main/.github/workflows/release.yml.

• Removed Test PyPI — no longer publish to Test PyPI and install from there; instead, install directly from the locally built wheel to validate.
• Two-job structure: build-and-test (find release branch, build, install from wheel, run import tests) -> publish-to-pypi (download artifacts, publish).
• Manual approval gate via GitHub Environment pypi-release with required reviewers — publishing pauses until a maintainer approves.
• OIDC trusted publishing using pypa/gh-action-pypi-publish — replaces Twine username/password secrets.
• Fixed deprecated ::set-output — no longer needed since branch resolution happens in the same job.

I have taken care of the following config setups to make it work:

• Create a GitHub Environment called pypi-release with required reviewers (Dhruv, Yiyi, and myself).
• Configure PyPI trusted publishing for the huggingface/diffusers repo, specifying the pypi-release environment.

[DN6]

Small nit. Running commands like this with setup.py is deprecated in favour of just running a single
python -m build which gives you both the sdist and the wheel
https://setuptools.pypa.io/en/latest/deprecated/commands.html#running-setuptools-commands

Think we can just use that here.

[sayakpaul]

Oh nice one. Updated.