CI: Build and upload content for Steam app 1070560 (SteamLinuxRuntime)
Ever since we started using the "layered" scout-on-soldier runtime (depot version 0.20210630.32, Steampipe build ID 6958552), the content of SteamLinuxRuntime has depended only on files shipped by steamrt/steamlinuxruntime>, which was merged into this project in v0.20221208.0.
This means it's no longer useful to build SteamLinuxRuntime as part of the same workflows as the soldier and sniper runtimes, so we can simplify its workflow by having its version advance in lockstep with pressure-vessel.
Helps: steamrt/tasks#200, steamrt/tasks#207