ci: fix check for Ubuntu 20.04

ci: fix check for Ubuntu 20.04

In 5ca0c455f1 (ci: fix Python dependency on Ubuntu 24.04, 2024-05-06),
we made the use of Python 2 conditional on whether or not the CI job
runs Ubuntu 20.04. There was a brown-paper-bag-style bug though, where
the condition forgot to invoke the `test` builtin. The result of it is
that the check always fails, and thus all of our jobs run with Python 3
by accident.

Fix this.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>