docs: github actions example: remove unneeded install step (#10356)
* Previously had uv python install, then uv sync --all-extras --dev
* If we are going to use sync for dev dependencies, then the install
step is unneccessary
<!--
Thank you for contributing to uv! To help us out with reviewing, please
consider the following:
- Does this pull request include a summary of the change? (See below.)
- Does this pull request include a descriptive title?
- Does this pull request include references to any relevant issues?
-->
## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
## Test Plan
<!-- How was it tested? -->
