setup-backend (used in 17 job steps across 13 workflows) bootstrapped uv
via `pip install --upgrade pip setuptools wheel uv` on every run, and only
cached pip's download dir via actions/setup-python's `cache: pip` -- pip
wasn't doing the actual installs (uv was), so that cache bought nothing.
astral-sh/setup-uv ships a prebuilt uv binary instead of installing it via
pip, and its own GitHub Actions cache (enable-cache: true) persists uv's
resolution/wheel cache across runs, keyed on requirements/*.txt and
pyproject.toml by default. Also drops the now-redundant pip/setuptools/wheel
upgrade -- uv's build isolation installs anything a package's build backend
declares (pyproject.toml's [build-system] already requires setuptools/wheel)
without needing them preinstalled in the target environment.
Also swaps the same `pip install uv` bootstrap in bump-python-package.yml.
The uv step is skipped when install-superset: false (helm-lint-test), since
otherwise there's nothing for it to install or cache and it fails the job.
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