BRP Python Bytecompile: Avoid hardcoding /usr/bin prefix for python
    
    Avoid using the full path and instead rely on PATH being correctly set
    up to find the executable.
    
    This fixes byte compilation for python2.7 when doing flatpak module
    builds where python2.7 can be in either /usr/bin or /app/bin, depending
    on how it's compiled.