if a build directory is accessible to other users it is possible to
smuggle data in and out of build directories. usually this ins only
a build purity problem, but in combination with other issues it can
be used to break out of a build sandbox. to prevent this we default
to using a subdirectory of nixStateDir (which is more restrictive).
Fixes CVE-2025-52991.
Change-Id: Iacfc9b50534de158618c815f9fb99d7dae1be4d0
I think it is bad for these reasons when `tests/` contains a mix of
functional and integration tests
- Concepts is harder to understand, the documentation makes a good
unit vs functional vs integration distinction, but when the
integration tests are just two subdirs within `tests/` this is not
clear.
- Source filtering in the `flake.nix` is more complex. We need to
filter out some of the dirs from `tests/`, rather than simply pick
the dirs we want and take all of them. This is a good sign the
structure of what we are trying to do is not matching the structure
of the files.
With this change we have a clean:
```shell-session
$ git show 'HEAD:tests'
tree HEAD:tests
functional/
installer/
nixos/
```
(cherry picked from commit 68c81c737571794f7246db53fb4774e94fcf4b7e)
