localrepo: change shebang due to moved env in OpenWrt 21.xx

In Turris OS 6.0, which is running OpenWrt 21.xx, there was moved /usr/bin/env to /bin/env. This binary is provided by busybox.

While looking in binaries written by us in /usr/bin, I see that we are using /usr/bin/python3 instead of /usr/bin/env and according to PEP 0394 [1], it should work, too.

[1] https://www.python.org/dev/peps/pep-0394/

Fixes: https://gitlab.nic.cz/turris/turris-os-packages/-/issues/735