build-sys: use full path to python in scripts

Commit c4eb236a2c didn't take into account the situation when the user
sets e.g. PYTHON=python3 (without the full path).  This value would
then be used verbatim for PYTHON_BINARY and in she-bang lines in
scripts, which is incorrect. To fix this, $PYTHON is passed through
which, which expands the path.  If $PYTHON_BINARY is desired which is
not installed on the build system, then PYTHON_BINARY must be set
separately.