refactor(ci): use astral-sh/setup-uv with vars.UV_VERSION across all gh workflows

[danceratopz]

#1061 experimented with the astral-sh/setup-uv action.

This has some nice features:

• specify uv version,
• specify python version to use,
• persist uv's cache between runs.

We main task is to use this action consistently across all workflows and ensure vars.UV_VERSION is used consistently.

Additionally, as this step's config is a few lines that'll get heavily repeated:

execution-spec-tests/.github/workflows/tox_verify.yaml

Lines 13 to 19 in 7d4f9b9

	- name: Install uv ${{ vars.UV_VERSION }} and python ${{ vars.DEFAULT_PYTHON_VERSION }}
	uses: astral-sh/setup-uv@v5
	with:
	enable-cache: true
	cache-dependency-glob: "uv.lock"
	version: ${{ vars.UV_VERSION }}
	python-version: ${{ vars.DEFAULT_PYTHON_VERSION }}

we could consider adding this to .github/actions/setup-uv/action.yaml (instead of using a curl install, as it does now). This action could optionally take a PYTHON_VERSION and default to vars.DEFAULT_PYTHON_VERSION.