Without the change the test build attempt fails the evaluation as:
$ nix build --no-link -f. distrobuilder.tests.incus
error: attribute 'container' missing
at pkgs/tools/virtualization/distrobuilder/default.nix:54:19:
53| passthru = {
54| tests.incus = nixosTests.incus.container;
| ^
This started happening after `container` test was split in two in
c607e70f70 "nixosTests.incus: add test with old and new init".
6eef1793ca