niri-visual-tests break nix

[KiaraGrouwstra]

$ nix flake check         
error: builder for '/nix/store/chiy80k6h8nxyfibw6h03n18wa4qfjdn-niri-deps-a388c257951107c6d45987bb390f312e5398da71.drv' failed with exit code 101;
       last 10 log lines:
       > ++ command cargo --version
       > cargo 1.76.0-nightly (26333c732 2023-11-28)
       > ++ command cargo check --release --locked --all-targets
       > error: failed to load manifest for workspace member `/build/source/niri-visual-tests`
       >
       > Caused by:
       >   failed to read `/build/source/niri-visual-tests/Cargo.toml`
       >
       > Caused by:
       >   No such file or directory (os error 2)

a git bisect start main 236abd9 && git bisect run nix flake check blames this on 122afff.

[YaLTeR]

This is where I need to enlist help of the nix people because visual tests should not be built for regular use and should be built for development