Review finding: with the install's own venv activated (a re-run), `uv python
find <range>` returned venv/bin/python3, which setup_venv then deleted before
handing the dead path to `uv venv`; the stage still exited 0 and printed
"Virtual environment ready". Probe with --system so only base interpreters
qualify, and fail the stage when the venv was not created.
check_python only asked uv for 3.11 and, missing that, downloaded it —
a hard failure on hosts that cannot reach GitHub releases even when a
3.12/3.13 inside requires-python is already installed. Ask uv for the
supported range second and pin the venv onto that interpreter.
Same direction as #10824 (@gnanirahulnutakki), reduced to the shell
path; the PowerShell installer already has this fallback.
Refs #10778
teknium1