Add github actions workflow (with caching)

[Peter0x44]

This is a continuation of #72, the changes I added were to make caching work, so the CI doesn't have to download tarballs on every run.

I am not sure if the changes to the Dockerfile are necessary. I initially made them because exporting the image to layers or something like that was hanging, and it fixed that. Docker is not something I have a lot of knowledge about. My changes were LLM authored, but I did test them as thoroughly as I could.

They are not. Removed.

[Peter0x44]

The changes to the Dockerfile seem to be required. I removed them in https://github.com/Peter0x44/w64devkit/actions/runs/18278436300/job/52035735401 and it hung.

[Peter0x44]

I managed to remove the changes to the Dockerfile, it still works.

[Peter0x44]

OK. There are still a few issues with this.

for PRs, it runs 2x the builds it needs to.

this might just be an issue with docker in general, but I had to update one of gcc's patches, but it did not invalidate the cache for x-gcc and continued using the previously cached build.

[Peter0x44]

I fixed the duplicate runs on pull requests. Should be good to merge @skeeto .

[Peter0x44]

No. I can't get that working.
But at least it cancels the jobs quickly after they start.

[Peter0x44]

@skeeto any objections left? I would love to get this in, I'm using it heavily.
It's being used in all of my experimental/research branches.