Use ubuntu-20.04 instead of ubuntu-latest
Use ubuntu-20.04 instead of ubuntu-latest

ubuntu-latest is now ubuntu-22.04:
https://github.blog/changelog/2022-11-09-github-actions-ubuntu-latest-workflows-will-use-ubuntu-22-04/

Signed-off-by: Kamil Rakoczy <539d6a2650324aa9156edbbb09228193dfee5b9b@antmicro.com>

alainmarcel/Surelog,alainmarcel/Surelog,chipsalliance/Surelog,alainmarcel/Surelog,chipsalliance/Surelog,chipsalliance/Surelog,alainmarcel/Surelog,chipsalliance/Surelog