The build fails when pyenv is present, as reported in report and also in #32749/#33103.

We fix it by rejecting the pyenv shims for sage-bootstrap-python.

Tickets:

• sage --gdb not working #29461 sage --gdb not working
• SAGE_ROOT/tox.ini: Add variants that provision a system python using pyenv #32531: SAGE_ROOT/tox.ini: Add variants that provision a system python using pyenv

CC: @embray @mkoeppe @slel @tobiasdiez @jhpalmieri

Component: build: configure

Author: Matthias Koeppe

Branch/Commit: 376932f

Reviewer: John Palmieri

Issue created by migration from https://trac.sagemath.org/ticket/29285