Makefile.uk: Ensure consistent Python binary name

Makefile.uk: Ensure consistent Python binary name

Previously Makefile.uk used `python` and `python3` interchangeably which
caused unexpected issues on misconfigured systems where these two do not
point to the same binary.
This change fixes this oversight by using `python` everywhere.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Radu Nichita <radunichita99@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
GitHub-Closes: #22