Improvements to the release process/CI

[denschub]

There are a couple of things I could improve.

• The binary inside the archive doesn't have the executable flag set. This is due to an issue in the upload-artifact action, but this can be worked around by building a tar/zip manually and uploading that.
• Instead of camo-linux-amd64.zip, it probably makes sense to add the version number to the archive, something like camo-v0.3.0-linux-amd64.zip so it's clear when linking to it.
• For releasing, I currently have to wait until the build job for the release branch is done, then downloading the artifacts, drafting a release, and attaching the artifacts there. In theory, I could run GHA when a new tag is created, build release artifacts, and automatically create a new draft release with the artifacts attached. The matrix-org/synapse repo has something close to that where I could probably take inspiration from.
• My current version number hacks based on ::set-output are deprecated, and there is a much nicer way available.
• action-rs seems unmaintained. This will become an issue soon since GitHub Actions will deprecate NodeJS 12. I can probably just manually use rustup and call cargo...

Optionally, some other things:

• The docker-release workflow currently doesn't care about the "pre-release" checkbox in a release, and would tag even pre-releases to latest. This probably isn't a big deal since I don't plan on releasing pre-releases, but it might be worth adjusting that, and only push a tag 1.0.0-dev, instead of pushing it to 1, 1.0, 1.0.0-dev, and latest.
• I currently push container images to Docker Hub, but there's no reason not to push it towards the GitHub Container Registry as well.

[denschub]

I've made some of the required changes (see the checklist in the initial issue), but I'll postpone some others, mainly the "maintainer comfort" things. GitHub Actions isn't my favorite CI, and I feel like to comfortably do this without too many hacks, I'd need to invent my own action(s). This isn't needed for a 1.0.0 release, so I'll unblock the release.

[denschub]

I'm currently too lazy to address any of the remaining issues, mainly because I don't have a need for now.