Added packages to release for ubuntu-20.04 only.

[YoshikiTakashima]

Description of changes:

This PR modifies the release workflow to also push docker images to GHCR. By having these images up in GHCR, users can access kani without compiling it.

Resolved issues:

Resolves #1587

Call-outs:

• Since running docker images in CI is restricted to linux, I only implemented the image for ubuntu 20.04
• During testing, make sure to have the

Testing:

• How is this change tested? Fork the PR branch, then do a fake release to check that the image is uploaded correctly.

• Is this a refactor change? No

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[celinval]

I'm assuming you performed the steps you described in the tests section, right?

[YoshikiTakashima]

@celinval At the time of the PR, it was untested b/c I wanted to get feedback quickly. Now it's tested

• Image: https://github.com/YoshikiTakashima/kani/pkgs/container/kani-ubuntu-20.04
• release: https://github.com/YoshikiTakashima/kani/releases/tag/kani-0.9.0
• action that made it: https://github.com/YoshikiTakashima/kani/runs/8047605724?check_suite_focus=true