fix(nix): fix runtime error of runtime difftest ld

[Clo91eaf]

This PR fix the problem of LD generated when running Xiangshan difftest by adding $LD_LIBRARY_PATH environment variable.

The reason is that glibc used in nix shell lacks libz.so related dependencies

For example, you can execute:

❯ ldd ./XiangShan/ready-to-run/riscv64-nemu-interpreter-so
        linux-vdso.so.1 (0x00007c39ecb48000)
        libz.so.1 => not found
        libstdc++.so.6 => not found
        libm.so.6 => /nix/store/g3s0z9r7m1lsfxdk8bj88nw8k8q3dmmg-glibc-2.40-66/lib/libm.so.6 (0x00007c399cf17000)
        libgcc_s.so.1 => /nix/store/fdsndp18qa1fk375vcd4vrn00c0p4zpr-xgcc-14.2.1.20250322-libgcc/lib/libgcc_s.so.1 (0x00007c39ecb14000)
        libc.so.6 => /nix/store/g3s0z9r7m1lsfxdk8bj88nw8k8q3dmmg-glibc-2.40-66/lib/libc.so.6 (0x00007c399cc00000)
        /nix/store/g3s0z9r7m1lsfxdk8bj88nw8k8q3dmmg-glibc-2.40-66/lib64/ld-linux-x86-64.so.2 (0x00007c39ecb4a000)

At the same time, this PR also made some simple changes to the nix script

[Clo91eaf]

@Tang-Haojin @ngc7331 Can you please take a look?

[ngc7331]

flake.nix change LGTM, thanks for your contribution!

Two questions regarding .gitignore change:

1. Why adding results/ to .gitignore?
2. Please make sure it's end with an empty line.

[Clo91eaf]

results/ are introduced because I am generating the output of some derivation with nix build while testing something. This does not occur in normal workflows, I have removed it from the commit. Thanks for the suggestions!