Prepare GitHub Actions Workflow for installing rsyslog's Ubuntu PPA-provided packages

[atc0005]

Base goals (as I understand them):

• Install packages for Ubuntu 16.04 LTS
• Install packages for Ubuntu 18.04 LTS

Extended goals:

• Install packages for Ubuntu 20.04 LTS
  • does rsyslog support Ubuntu 20.04 yet?
• Run config validation?
• Run testbench?

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• Develop some type of CI/QA for the packages rsyslog/rsyslog-pkg-ubuntu#69

[atc0005]

Example of the cron schedule syntax here:

• https://github.com/cloudalchemy/ansible-grafana/blob/master/.github/workflows/labeler.yml

Examples of the standard workflow that I use for Go-based builds:

• https://github.com/atc0005/check-cert/blob/master/.github/workflows/lint-and-build-code.yml
• https://github.com/atc0005/check-cert/blob/master/.github/workflows/lint-docs.yml

[atc0005]

Upstream PR accepted, work complete.