[fix] re-add 'pip-exe' target - partial revert 9b48ae47

Target pip-exe is a prerequisite of the targets:

  - pyinstall
  - pyuninstall

and was accidentally deleted in commit 9b48ae47.

HINT:
  do not confuse pyinstall with penvinstall

pyinstall & pyuninstall
    Installing into user's HOME using pip from OS,
    therefore the message is needed.

pyenvinstall & pyenvuninstall
    Installing into virtualenv (./local) using pip which is provided by
    prerequisite 'pyenv' in the virtualenv.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>