Closes #23342 

Ran `nix flake update` and did some cleanup in shell.nix to follow nix
[best
practices](https://discourse.nixos.org/t/how-to-solve-libstdc-not-found-in-shell-nix/25458/6).

Prior to running `nix flake update`
`strings "$(echo "$LD_LIBRARY_PATH" | tr : "\n" | grep
"gcc")/libstdc++.so.6" | grep "CXXABI_1.3.15" CXXABI_1.3.15`
Does not find `CXXABI_1.3.15`

After running `nix flake update`

`strings "$(echo "$LD_LIBRARY_PATH" | tr : "\n" | grep
"gcc")/libstdc++.so.6" | grep "CXXABI_1.3.15" CXXABI_1.3.15`
Finds `CXXABI_1.3.15`

Launching Zed 0.168.3 inside Zed's nix development shell now launches
with no errors.

Release Notes:

- N/A