nix: Update nix flake

[AidanV]

Closes #23342

Ran nix flake update and did some cleanup in shell.nix to follow nix best practices.

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

[maxdeviant]

Thank you!