Fix: uploading of GRFcodec releases

[rubidium42]

It would be preferable to just have GRFcodec releases at the "old" location, but the upload part has been lost when migrating to GitHub. This is a counterpart to OpenTTD/website#319, where this would be the better solution.

I copied bits from OpenTTD's code but I have no idea how to test this as I do not have access to any of the secrets required for uploading.

I hope @TrueBrain or @glx22 might be able to test whether this actually uploads and especially whether it uploads it to the right location. The right location being https://www.openttd.org/downloads/grfcodec-releases/latest for releases and https://www.openttd.org/downloads/grfcodec-nightlies/latest for the nightlies. Otherwise it'll be a lot of YOLO testing in production with nightlies I fear.

[TrueBrain]

A copy/paste should be fine, yes. So this looks good on first sight. There are two things we do need to take care of:

      CDN_SIGNING_KEY: ${{ secrets.CDN_SIGNING_KEY }}
      DEPLOYMENT_APP_ID: ${{ secrets.DEPLOYMENT_APP_ID }}
      DEPLOYMENT_APP_PRIVATE_KEY: ${{ secrets.DEPLOYMENT_APP_PRIVATE_KEY }}

These are the three secrets this change needs. The last two are easy, as they are the same for all OpenTTD repository. The first one however, is a bit more tricky. I need to add somewhere that this repo has access to upload to the above mentioned folders. I am not completely sure anymore where I had to do that .. I will check that out later this week. If I forget, poke me.

[TrueBrain]

From what I can tell, the source job in this workflow doesn't define either of those three. For reference:

https://github.com/OpenTTD/OpenTTD/blob/master/.github/workflows/release-source.yml#L145

It isn't completely trivial to create the right variables, as it needs to detect whether it is a nightly or release.

[glx22]

I don't think we have any artifacts as gh is used to directly upload bundles in the release assets.