Fix Main Workflow

[sdimitro]

Description

It seems like Github changed the ubuntu-latest OS from 18.04 to 20.04
and Python 3.6 and 3.7 are not part of it. We are currently still in 18.04 though
so we may as well freeze that version.

BTW this will unblock our testing for the PRs created by our automated actions
create to sync 6.0/stage with master.

Testing

The main workflow passes again as seen in this PR.

[prakashsurya]

out of curiosity, why replace "apt-get" with "apt"?

[sdimitro]

out of curiosity, why replace "apt-get" with "apt"?

Honestly I didn't think too much about this. I added the apt update line and then decided to change the other packaging command in that action to apt from apt-get for what it felt as consistency at the time.

Reading up on the differences, it seems like apt is just a higher-level wrapper over the most common apt-get, apt-cache, etc.. commands. I personally have switched to using apt when I'm using the shell interactively so I don't need to remember "use apt-get for update, install and apt-cache for search, show".

I don't feel strongly about it so I'd be happy to use apt-get if you think it is better.

[prakashsurya]

I don't mind either way. I often use the other commands, but using apt is fine too. Doesn't really matter, just was curious.. I'm good as-is.