Steps To Reproduce

On a system where python3==python3.14, one cannot force the build to honour
--with-python=python3.13 (or 3.12) even if python3.13 (or 3.12) is installed and good to use.

This is a blocker, a bug in the logic of build/pkgs/python3/spkg-configure.m4

Or, rather, the bug is in how the results of ./configure are used. By right, PYTHON_FOR_VENV should be the one used while running ./sage --python, but this does not happen in the situation here.

Expected Behavior

python3.x should be chosen.

Actual Behavior

python3 is chosen instead

Additional Information

No response

Environment

• OS: macOS, Linux
• Sage Version: 10.8.rc0

Checklist

• I have searched the existing issues for a bug report that matches the one I want to file, without success.
• I have read the documentation and troubleshoot guide