better usage of UV_PRERELEASE=allow

[sayakpaul]

What does this PR do?

The huggingface_hub RC-testing bot (which produces branches like ci-test-huggingface-hub-1.14.0.rc0-release) rewrites every uv pip install in our workflows to uv pip install --prerelease=allow via a text substitution. On lines that already carried --prerelease allow (added in #12395 so non-RC CI can still install transformers-from-main when it depends on a hub RC), this produced:

uv pip install --prerelease=allow --prerelease allow -U transformers@git+...

uv rejects this:

error: the argument '--prerelease <PRERELEASE>' cannot be used multiple times

Example: https://github.com/huggingface/diffusers/actions/runs/25439680072/job/74627420559

Switching to the UV_PRERELEASE=allow env-var prefix is equivalent and fixes the problem.