ci: ignore APT update failure in GitHub Actions

ci: ignore APT update failure in GitHub Actions

Ubuntu 18.04 GHA virtual machine images point at an invalid APT
repository.
We have no control over this, simply ignore the failure.

This was caught by Ilya for OVS and the robot just hit the same issue
for DPDK:

"""
Get:46 http://security.ubuntu.com/ubuntu bionic-security/restricted
Translation-en [29.9 kB]
Get:47 http://security.ubuntu.com/ubuntu bionic-security/universe amd64
Packages [1104 kB]
Get:48 http://security.ubuntu.com/ubuntu bionic-security/universe
Translation-en [247 kB]
Reading package lists...
E: The repository 'https://apt.postgresql.org/pub/repos/apt bionic-pgdg
Release' no longer has a Release file.
Error: Process completed with exit code 100.
"""

Fixes: 9d620630ea30 ("ci: fix package installation in GitHub Actions")

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Ilya Maximets <i.maximets@ovn.org>
Acked-by: Aaron Conole <aconole@redhat.com>