ci: expire artifacts

This MR introduces a small improvement to our CI configuration.

The artifacts generated by each pipeline are roughly 250MB and they are only needed for testing the build in gitlab-web-ide before releasing: https://gitlab.com/gitlab-org/gitlab-web-ide/-/tree/main/docs/dev#setup-integrating-with-candidate-vs-code-build

This MR makes the artifacts expire in 10 days.

We were also needlessly exposing artifacts from the publish job. This MR removes that.

I tested that this new configuration works in my personal fork of this project: https://gitlab.com/viktomas/gitlab-web-ide-vscode-fork/-/pipelines/659806766

Relates to gitlab#373490 (closed)