Running ./bootstrap resulted in several warnings of the form

/mnt/d/Programming/sage/src/bin/sage: /mnt/d/Programming/sage/src/bin/sage-venv-config: /doesnotexist/python3: bad interpreter: No such file or directory

This is fixed by using the currently active python instead.

CC: @mkoeppe

Component: build

Author: Matthias Koeppe

Branch/Commit: d97109e

Reviewer: Tobias Diez

Issue created by migration from https://trac.sagemath.org/ticket/31218