Without this change, it is challenging to create integration tests to verify that the folders not included in `writable_roots` in `SandboxPolicy::WorkspaceWrite` are read-only because, by default, `get_writable_roots_with_cwd()` includes `TMPDIR`, which is where most integrationt tests do their work. This introduces a `use_exact_writable_roots` option to disable the default includes returned by `get_writable_roots_with_cwd()`. --- [//]: # (BEGIN SAPLING FOOTER) Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/openai/codex/pull/1785). * #1765 * __->__ #1785