Use distro_python_version in fernet-node-sync

Since Debian and Ubuntu are already on Python3 only and don't have unversioned
Python binaries (no /usr/bin/python) - we need to call the fetch-fernet-tokens
script using distro_python_version

Backport: train
Related-Bug: #1859047
Change-Id: I42378af9b25f14079fc57b4068ab25d5d4877362