Use preinstalled djgpp image in workflows

[volkertb]

Hopefully this will speed up the pipelines, since DJGPP doesn't have to be downloaded and installed in each pipeline when they are run on this image that already includes a DJGPP installation.

Also, using specific image versions should make the builds fully reproducible.

For the project that builds and publishes this image, see https://github.com/volkertb/debian-djgpp

[volkertb]

This line should be removed before merging this PR.

[thp]

This workflow could also be run for all PRs against main, but skip the Github releases part (using conditions) if we're not on main?

[thp]

You need to modify the container image, not the runner.

[volkertb]

@thp @crazii I'm closing this PR, at least for the time being. I need to rethink part of this, since I would have to include certain tools in the builder image, specifically for building and packaging this project (wget, git, shellcheck, perhaps others I've missed so far).

The best course of action:

• Offer a PR to https://github.com/andrewwutw/build-djgpp that builds and publishes a container image whenever a new version of DJGPP is released. An additional advantage of this is that the image would be "owned" by the same GitHub user that maintains the DJGPP repository on GitHub (andrewwutw).
• Create a separate GitHub project that builds and publishes an SBEMU-specific build container image that uses that official DJGPP image as its base image.
• Create a new PR in the SBEMU project to use that SBEMU-specific build image in the GitHub Actions workflows.

Of course I'm open to any suggestions for further improvements, as well as any simpler alternatives that could provide comparable benefits (primarily faster builds).