Replaces the `include_default_writable_roots` option on `sandbox_workspace_write` (that defaulted to `true`, which was slightly weird/annoying) with `exclude_tmpdir_env_var`, which defaults to `false`. Though perhaps more importantly `/tmp` is now enabled by default as part of `sandbox_mode = "workspace-write"`, though `exclude_slash_tmp = false` can be used to disable this.