return the full path of 'pip' executable if found.
pip_exec()executable full path in a character vector of length 1.
Other executable full path:
chrome_exec(),
excel_exec(),
firefox_exec(),
libreoffice_exec(),
node_exec(),
npm_exec(),
powerpoint_exec(),
python_exec(),
word_exec()
if(exec_available("pip"))
message(pip_exec())
#> /opt/python/3.12.4/bin/pip3