add tests for ubuntu 20.04

[emmettbutler]

This change adds Ubuntu 20.04 arm and amd64 to the list of operating systems tested for single-step instrumentation compatibility.

Reviewer checklist

• If PR title starts with [<language>], double-check that only <language> is impacted by the change
• No system-tests internal is modified. Otherwise, I have the approval from R&P team
• CI is green, or failing jobs are not related to this change (and you are 100% sure about this statement)
• A docker base image is modified?
  • the relevant build-XXX-image label is present
• A scenario is added (or removed)?
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[robertomonteromiguel]

We should set this new vm as default for now

[robertomonteromiguel]

`Diagnostics:
pulumi:pulumi:Stack (system-tests-vms-system-tests_onboarding):
error: update failed

aws:ec2:Instance (Ubuntu_20_amd64):
error: 1 error occurred:
* creating EC2 Instance: OptInRequired: In order to use this AWS Marketplace product you need to accept terms and subscribe. To do so please visit https://aws.amazon.com/marketplace/pp?sku=9rxhntdy981dz5t3gbzpdd60w
status code: 401, request id: a61747bb-600c-4e24-a23c-00ebfe4ecfa1
`