Remove NIXPATH references; bump gitignore.nix

[madeline-os]

Since we upgraded obelisk to use nix-thunk, which was also recently upgraded to avoid referencing the NIXPATH via <nixpkgs>, we can make obelisk just a little purer by upgrading the thunks. In the tests, we use a pinned nixpkgs so that even our VM tests do not have this impurity. The self-tests for legacy thunks, which necessarily use <nixpkgs> are now the only references to it in the code.

gitignore.nix upstream has also recently fixed a source of impurity, so we bump that dependency.

This is a modest step towards pure-eval and eventually, flakes support.

See #854 for discussion

TODO

remove impurity in Obelisk.Command.VmBuilder

I have:

• [x] Based work on latest develop branch
• [x] Followed the contribution guide
• [x] Looked for lint in my changes with hlint . (lint found code you did not write can be left alone)
• [x] Run the test suite: $(nix-build -A selftest --no-out-link)
• [x] Updated the changelog
• [x] (Optional) Run CI tests locally: nix-build release.nix -A build.x86_64-linux --no-out-link (or x86_64-darwin on macOS)

[exarkun]

is there something I can do to help move this forward? I would like to use obelisk from a flake without resorting to impure mode.