Nix build does not evaluate due to breaking changes in fetchTree

[mstrangfeld]

Currently, when you try to build Helix with Nix, you will get this error:

$ nix build .#helix
error:
       … while calling the 'derivationStrict' builtin
         at <nix/derivation-internal.nix>:9:12:
            8|
            9|   strict = derivationStrict drvAttrs;
             |            ^
           10|

       … while evaluating derivation 'helix-term-23.10.0'
         whose name attribute is located at /nix/store/bjvqq8c79dbi59g7xzcc6lhl0f19m3d7-source/pkgs/stdenv/generic/make-derivation.nix:353:7

       … while evaluating attribute 'buildCommand' of derivation 'helix-term-23.10.0'
         at /nix/store/bjvqq8c79dbi59g7xzcc6lhl0f19m3d7-source/pkgs/build-support/trivial-builders/default.nix:98:17:
           97|         enableParallelBuilding = true;
           98|         inherit buildCommand name;
             |                 ^
           99|         passAsFile = [ "buildCommand" ]

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: in pure evaluation mode, 'fetchTree' will not fetch unlocked input 'github:tree-sitter/tree-sitter-agda/c21c3a0f996363ed17b8ac99d827fe5a4821f217'

This is due to this breaking change in how the fetchTree for GitHub works: NixOS/nix@09d76e5

To fix this, we need to compute the NAR hash for each grammar first to verify the integrity of the GitHub tarballs.