Fix nix gcc vanishing paths

[ConnorNelson]

We shouldn't have /nix/store/... paths in our ELFs. Our nix stores are not stable, we might garbage collect them at any point. This is largely separate from challenge authors compiling in the dojo and uploading the created files (we should find a smoother development process that sidesteps this entirely). Instead, this has to do with the idea that a user might compile a program in their home directory, and that program should continue to work in any challenge image. In this case, ld should actually be from nix, since we have no idea if the challenge image will have an ld. We still might run into issues related to ld and the libraries changing (upgrading for example), but we can be way more stable than the current /nix/store/... references.

Currently:

hacker@paths~the-root:~$ echo 'int main() { }' > test.c
hacker@paths~the-root:~$ gcc test.c -o test
hacker@paths~the-root:~$ ldd test
	linux-vdso.so.1 (0x00007ffc6f3ed000)
	libc.so.6 => /nix/store/r8qsxm85rlxzdac7988psm7gimg4dl3q-glibc-2.39-52/lib/libc.so.6 (0x00007c13a6b52000)
	/nix/store/r8qsxm85rlxzdac7988psm7gimg4dl3q-glibc-2.39-52/lib/ld-linux-x86-64.so.2 => /lib64/ld-linux-x86-64.so.2 (0x00007c13a6d41000)
hacker@paths~the-root:~$ readelf -a test | grep nix
      [Requesting program interpreter: /nix/store/r8qsxm85rlxzdac7988psm7gimg4dl3q-glibc-2.39-52/lib/ld-linux-x86-64.so.2]
 0x000000000000001d (RUNPATH)            Library runpath: [/nix/store/r8qsxm85rlxzdac7988psm7gimg4dl3q-glibc-2.39-52/lib:/nix/store/qksd2mz9f5iasbsh398akdb58fx9kx6d-gcc-13.2.0-lib/lib]
hacker@paths~the-root:~$ strings test | grep nix
/nix/store/r8qsxm85rlxzdac7988psm7gimg4dl3q-glibc-2.39-52/lib/ld-linux-x86-64.so.2
/nix/store/r8qsxm85rlxzdac7988psm7gimg4dl3q-glibc-2.39-52/lib:/nix/store/qksd2mz9f5iasbsh398akdb58fx9kx6d-gcc-13.2.0-lib/lib

If we need an absolute path to ld for the interpreter (probably we do, that would make sense?) it should be something like /run/dojo/lib/ld-linux-x86-64.so.2 (on ubuntu:24.04 I see /lib64/ld-linux-x86-64.so.2). We can make sure libc and friends end up in /run/dojo/lib and point RUNPATH there.