CI: speed up Ubuntu runs

[kroening]

This is an attempt to speed up CI jobs on Ubuntu runners, which spend between 2 and 3 minutes on apt-get update and apt-get install.

[tautschnig]

Might work most of the time, but could break at some arbitrary point of time if there ever was a security update to those packages.

[kroening]

The question is whether Github would update the image often enough?

[tautschnig]

The question is whether Github would update the image often enough?

I believe that's almost always true, but I recall (in other repos) adding the apt-get update step to address CI failures for the occasions where that wasn't the case. How about the following:

packages_to_install="pkg1 pkg2 ..."
if ! sudo apt-get install $packages_to_install ; then
  sudo apt-get update
  # retry after updating package indices
  sudo apt-get install $packages_to_install
fi