Unbounded disk usage of workspaces when using omnix CI 'apps'

[srid]

Omnix, which we use to build, can do more than invoke nix commands. Here, running commands like cargo test (see here) can end up populating the ./target directory which escapes Nix's garbage collection.

[srid]

Related: since acid-state keeps the whole state in memory, the growing number of build workspaces can be a problem from memory consumption standpoint as well.

[srid]

Done in #269