Rework the repo to build edk2 release tags

[nicholasbishop]

The builder this repo used to pull from has been turned off. It would be nice to have up-to-date edk2 release builds for use with testing in https://github.com/rust-osdev/uefi-rs, without relying on the vagaries of Linux distro releases. Doing our own builds also allows us to ensure we have exactly the build options we want for use with VM testing.

As described in the readme, the intended usage is manually pushing a tag corresponding to a release tag in the edk2 repo. That tag will be built in a container, and the resulting tarball will be pushed as a release in this repo (as long as the release doesn't already exist.)

I've intentionally kept the container build very simple and linear to make it clear what options are being set for each edk2 build. That leads to some duplication there, but I think the end result is quite easy to follow.

Here's an example release from my testing in a fork: https://github.com/nicholasbishop/ovmf-prebuilt/releases/tag/edk2-stable202211-r3

[nicholasbishop]

@phil-opp any thoughts on this change? If you're OK with it I'll merge this and handle pushing tags to trigger release builds.

[phil-opp]

Thanks a lot for looking into this! I'm currently on vacation, but I'll take a closer look once I'm back. The approach looks good to me in general, so feel free to merge this already without my review.

[nicholasbishop]

Sounds good, thanks!