Rework venv activation

[eifinger]

Problem

If python-version is defined, setup-uv currently auto-activates a venv under .venv in the repository/workspace root.

Problems with this behavior:

• Does not support repos with multiple possible venvs: Support working-directory in action configuration #271

Other considerations:

• Interaction with the pyproject-file input? VIRTUAL_ENV is set to wrong directory when using pyproject-file config #251
• Interaction with .python-version file: Support .python-version files as input to with python-version #211
• Support --system-site-packages: Flag for --system-site-packages when initializing venv #260
• Should the correct python version be always installed? Python version part of the cache key is undefined if the required python version is not yet installed #283

Solutions

Solution 1

• a new boolean input activate-environment
• a new input working-directory which defaults to ${{ github.workspace }}. If activate-environment: true it will activate the venv in this place
• We automatically look for pyproject.toml and .python-version in working-directory

Optional: We always install the target python version (determined by python-version, pyproject.toml or .python-version).

Open question: What to do about --system-site-packages?