Draft: Nixos support

[RTUnreal]

This PR adds a nix package build via flake.nix.

Due to overte's dependency on node18, this PR is currently blocked until that is updated. This is due to node18, at least in nixpkgs, requires icu75, but everything else needs icu76.

Dependencies/before this PR:

• [x] #1781
• [ ] #1711
• [ ] #1848 (TODO)
• [ ] soft: updated glm
• [ ] soft: #1808

[RTUnreal]

Also to note as of a week ago, qt5.qtwebengine is marked as insecure, as it contains a lot of unfixed CVEs, due to qt5webengine not being maintained anymore upstream. 1