Create a public APT repository for `code-server`

The `.deb` packages for `code-server` are super handy, but users still have to manually update them by downloading and installing the fresh package for each release. It would be really nice to have an APT repository so this was handled automatically. (See #2600 and #2582 for some earlier discussion of the topic.)

Since the `.deb` packages are already being built, this should be quite easy to do with a host like [Buildkite Package Registry](https://buildkite.com/platform/package-registries/) (fka PackageCloud), which has a "free for open source" option. To test this out, I set up a registry with a personal fork, and it seems to work exactly as expected: https://buildkite.com/organizations/andrew-gainer/packages/registries/code-server/packages/019dee3f-dd00-7900-bf17-b6a98bedf40c

I'd be happy to help set this up for the main `coder/code-server` repo, but it's not something that can be done with just a pull request. The steps required would be:
* Set up a Buildkite organization for Coder with the "free for open source" plan and connect it to the `coder/code-server` repo
* Mint a Buildkite API token and add it as a repo secret
* Add code to the `release.yaml` GitHub action to upload the `.deb` archives to Buildkite
* Update the [coder.com docs](https://coder.com/docs/code-server/install) to reference the new repo

If y'all are interested in getting this set up, I'm happy to help!