pygrub: Set path in #! line of pygrub, too

pygrub: Set path in #! line of pygrub, too

pygrub currently has a hardcoded path of /usr/bin/python which is not
correct if the version of python at install time is not the same as
that at build time.  This patch uses the existing install-wrap and
python/get-path machinery.

(It does not address the currently-existing bug that the get-path
machinery works by assuming that `python' is a symlink, rather than
querying the python interpreter for its version.)

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>