test_hetrodyne_pulsar: uses `/usr/bin/env python` instead of Python found during configure

Building the latest LALApps release on EL7 fails as one of the test codes is using /usr/bin/env python instead of the Python found during the configure process:

https://koji.ligo-la.caltech.edu/kojifiles/work/tasks/8198/18198/build.log

As I workaround I will update it to use /usr/bin/env python3 which should allow things to build but this is just a workaround and not a general fix.