Stop committing built files and update GH Actions references

[helen]

Description of the Change

Update some GH Actions version references and run the build process in CI instead of committing built files.

Alternate Designs

n/a

Benefits

No more committed built files! Latest and greatest GH Actions for release/update!

Possible Drawbacks

If we add more stuff to the build process the asset updater might not be able to run successfully unless it is updated to work better with build processes in the meantime.

Verification Process

As usual, wait and see, though these are copied from existing working plugins.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

See #77 for a time where the built version was (understandably) not in the PR and was forgotten about.

Changelog Entry

Guess it needs one in the release branch?

[dinhtungdu]

What is the node version used in the build? I need to use node 10 to install and build assets for simple podcasting.

Need to clarify node version first