Avoid unnecessary pyenv usage in environment setup

[deepyaman]

Summary & Motivation

Resolve #26078

How I Tested These Changes

The change in instructions reflects how I set up my local environment. I haven't tested the Buildkite part yet.

Changelog

Avoided unnecessary pyenv usage in environment setup step of contributing guide.

[github-actions[bot]]

Deploy preview for dagster-docs ready!

Preview available at https://dagster-docs-9ya20d7wz-elementl.vercel.app https://deepyaman-docs-use-uv-venv.dagster.dagster-docs.io

Direct link to changed pages:

• https://dagster-docs-9ya20d7wz-elementl.vercel.app https://deepyaman-docs-use-uv-venv.dagster.dagster-docs.io/community/contributing
• https://dagster-docs-9ya20d7wz-elementl.vercel.app https://deepyaman-docs-use-uv-venv.dagster.dagster-docs.io/dagster-plus/deployment/serverless
• https://dagster-docs-9ya20d7wz-elementl.vercel.app https://deepyaman-docs-use-uv-venv.dagster.dagster-docs.io/integrations
• https://dagster-docs-9ya20d7wz-elementl.vercel.app https://deepyaman-docs-use-uv-venv.dagster.dagster-docs.io/integrations/airlift/federation-tutorial/setup
• https://dagster-docs-9ya20d7wz-elementl.vercel.app https://deepyaman-docs-use-uv-venv.dagster.dagster-docs.io/integrations/airlift/tutorial/setup

[neverett]

Tagging @alangenfeld and @smackesey since this is a change to the core engineering development process and not strictly a docs change.

[deepyaman]

Hmm... this seems to pass by and large (helm failure seems unrelated), but I also haven't figured out yet where changes to python_modules/automation/automation/docker/images/buildkite-test/Dockerfile are actually getting used. The actual tests seem to be getting run via tox config generated in .buildkite/dagster-buildkite/dagster_buildkite/steps/tox.py.