Potentially all python pkgs with `venv` are broken

[mxcl]

xpra currently has a hardcoded absolute path symlink to python that means it fails if we don’t install things to /opt:

$ ls -la /usr/local/pkgs/xpra.org/v6.2.3/venv/bin
lrwxrwxrwx 1 root   root     35 Jan 23 15:03 python -> /opt/python.org/v3.11.11/bin/python

Refs pkgxdev/pkgm#15