Add build for upcoming Ubuntu LTS 22.04

[tobiasherzke]

Ubuntu 22.04 will be released next month. It is unclear when github actions will add an ubuntu-22.04 runner.

This is just a suggestion how you could already now build and test your future 1.16 release for Ubuntu 22.04.

BTW you have random failures in one of your unit tests:

install/bin/lsl_test_exported --order rand --wait-for-keypress never --durations yes
5892 Segmentation fault      (core dumped) install/bin/lsl_test_exported --order rand --wait-for-keypress never --durations yes

I had this once affect the 18.04 build while the 20.04 build succeeded and once the other way round. I mention this here because this may also fail randomly in the builds triggered by this pull request.

[cboulay]

Thanks!