fix: update ubuntu-latest to point to ubuntu-20.04 (#580)

GitHub Actions switched `ubuntu-latest` to `ubuntu-20.04`
Ref:
  https://github.com/actions/virtual-environments/issues/1816
  https://github.com/actions/virtual-environments/pull/2852

README.md

@@ -139,12 +139,12 @@ If the `path:` value doesn't match the name of the repository, a `MODULE_NOT_FOU
 
 GitHub Actions offers managed [virtual environments](https://help.github.com/en/actions/reference/virtual-environments-for-github-hosted-runners) for running workflows. In order for `act` to run your workflows locally, it must run a container for the runner defined in your workflow file. Here are the images that `act` uses for each runner type and size:
 
-| GitHub Runner  | Micro Docker Image                 | Medium Docker Image                      | Large Docker Image                            |
-| -------------- | ---------------------------------- | ---------------------------------------- | --------------------------------------------- |
-| ubuntu-latest  | [node:12.20.1-buster-slim][micro]  | [catthehacker/ubuntu:act-latest][medium] | [nektos/act-environments-ubuntu:18.04][large] |
-| ubuntu-20.04   | [node:12.20.1-buster-slim][micro]  | [catthehacker/ubuntu:act-20.04][medium]  | `unavailable`                                 |
-| ubuntu-18.04   | [node:12.20.1-buster-slim][micro]  | [catthehacker/ubuntu:act-18.04][medium]  | [nektos/act-environments-ubuntu:18.04][large] |
-| ubuntu-16.04   | [node:12.20.1-stretch-slim][micro] | [catthehacker/ubuntu:act-16.04][medium]  | `unavailable`                                 |
+| GitHub Runner | Micro Docker Image                 | Medium Docker Image                      | Large Docker Image                            |
+| ------------- | ---------------------------------- | ---------------------------------------- | --------------------------------------------- |
+| ubuntu-latest | [node:12.20.1-buster-slim][micro]  | [catthehacker/ubuntu:act-latest][medium] | `unavailable`                                 |
+| ubuntu-20.04  | [node:12.20.1-buster-slim][micro]  | [catthehacker/ubuntu:act-20.04][medium]  | `unavailable`                                 |
+| ubuntu-18.04  | [node:12.20.1-buster-slim][micro]  | [catthehacker/ubuntu:act-18.04][medium]  | [nektos/act-environments-ubuntu:18.04][large] |
+| ubuntu-16.04  | [node:12.20.1-stretch-slim][micro] | [catthehacker/ubuntu:act-16.04][medium]  | `unavailable`                                 |
 
 Below platforms are currently **unsupported and won't work** (see issue [#97])