ipsec: Use @PYTHON@ directly instead of "/usr/bin/env python"

ipsec: Use @PYTHON@ directly instead of "/usr/bin/env python"

Using "/usr/bin/env" is against Fedora Packaging Guidelines [1].

Moreover, in this specific case, it also prevent "make rpm-fedora" to
successfully complete on "Fedora Rawhide" since "#!/usr/bin/env python"
must not be used anymore [2].

[1] https://fedoraproject.org/wiki/Packaging:Guidelines#Shebang_lines
[2] https://fedoraproject.org/wiki/Changes/Make_ambiguous_python_shebangs_error

CC: Qiuyu Xiao <qiuyu.xiao.qyx@gmail.com>
Fixes: 22c5eafb6efa ("ipsec: reintroduce IPsec support for tunneling")
Signed-off-by: Timothy Redaelli <tredaelli@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>