fix(ci): use the intended Python interpreter for CMake discovery (#6105)

* fix(ci): pin the discovered Python to the venv/setup-python interpreter

The manylinux/musllinux images added Python 3.15 and dropped 3.13t;
FindPython's version scan then picked /usr/local/bin/python3.15 over
the uv-created venv (Python_ROOT_DIR is only a hint). Similarly, the
macOS and Windows 3.14t jobs picked a system 3.14 over setup-python's
free-threaded interpreter. Pin the interpreter explicitly in both
places, and drop the 3.13t manylinux jobs.

Assisted-by: ClaudeCode:claude-fable-5

* ci: use setup-python's python-path output instead of a locate step

Assisted-by: ClaudeCode:claude-fable-5

* Apply suggestions from code review

Co-authored-by: Ralf W. Grosse-Kunstleve <rwgkio@gmail.com>

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Co-authored-by: Ralf W. Grosse-Kunstleve <rwgkio@gmail.com>