nix-build failed because the tests assume to run in a CWD equal to the
project root, which is not the case in the derivation build.
This commit fixes it by not using hacky `..` references to paths,
and instead uses NIX_PATH for all implicit Nix testing path
dependencies.
Also the root of the `lib` path gets passed in from the `default.nix`
file, so all the relative path handling is done by Nix during evaluation
already, and in the Nix store when possible.
- passing it as expression gives large error messages
which are not very readable
- this commits puts the file in nix-store
and patches the final program to have access to
the path to the file as env.
- We simply pass this file to nix-instantiate
We seem to have enough tests to run into this now:
error: creating symlink from '/private/tmp/nix-build-nixpkgs-check-by-name.drv-0/source/test-tmp/var/nix/gcroots/profiles' to '/private/tmp/nix-build-nixpkgs-check-by-name.drv-0/source/test-tmp/var/nix/profiles': File exists
