CI: use Python from uv's virtualenv

uv allows "uv python install" to a ~/.local, but doesn't allow
"uv pip install --user", only "--system". According to [1], we are
supposed to use a virtualenv, and make sure that "$VIRTUAL_ENV/bin"
is in "$PATH" (before "/bin").  Use ~/.local; there should be no
collisions in practice.

This fixes the problem worked around by fe3c42af9e (Work around
github actions python failure, 2026-02-08). Revert that.
Update lock files with:

	updatecli apply --config updatecli.d/python.yml
	uv lock --upgrade --exclude-newer="$(awk -F'"' <uv.lock '/^exclude-newer[[:space:]]*=/ {print $2}')"

[1]: https://github.com/astral-sh/uv/issues/2077#issuecomment-1971579880

updatecli.d/python.yml

@@ -5,7 +5,6 @@ sources:
     kind: shell
     spec:
       command: |
-        # TODO github actions might not have this
         build_tools/version-available-in-debian.sh stable python3-defaults
   sphinx_version:
     kind: shell