ci/test: run jobs on ubuntu-20.04, instead of ubuntu-18.04

[umarcor]

Description

Since the base image of the Dockerfile is to be updated to ubuntu:focal (see #1970), this PR changes the environment used in the jobs of workflow 'test'.

Related Issue

• Fixed root Dockerfile for building vtr container #1970

Motivation and Context

Ubuntu 18.04 is maintained until April 2023 (see https://endoflife.software/operating-systems/linux/ubuntu), however, Ubuntu 20.04 is also LTS. Hence, it is sensible to bump the environment, despite 18.04 not being EOL yet.

How Has This Been Tested?

It's CI, so self tested.

Types of changes

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

[mithro]

While we don't have to run everything on it, we should have at least some running jobs on the oldest OS version we support?

[umarcor]

@mithro, I believe there is an specific job/matrix for that: https://github.com/verilog-to-routing/vtr-verilog-to-routing/blob/master/.github/workflows/test.yml#L278-L310. So, the purpose of "Compatibility" is to evaluate the build on multiple OS versions. On the other hand, the rest of the jobs are all executed on the same (ubuntu-18.04 now, ubuntu-20.04 proposed).

[umarcor]

@vaughnbetz @mithro can we have this merged?