unigine-valley: refactor

[kierdavis]

Motivation for this change

I was quite inexperienced with Nix/NixOS when I wrote this expression a couple of months ago. This PR cleans up the expression to be more consistent with other Nix expressions.

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

---

Changes:

• Remove unnecessary and un-Nix-y "release version".
• Only run strip over the directory containing binaries and shared libraries, rather than the entire installation directory.
• Change install location from $out/opt/unigine/valley to $out/lib/unigine/valley.
• Define the above install location in a derivation variable, so that it doesn't have to copy-pasted everywhere.
• Remove debug messages from ELF patching phase.
• Clean up some comments.

There are a few other things that I'd like to fix in the near future, but need some more input:

• The upstream package should work on 32-bit and 64-bit x86 systems; however I think this expression will only work on 64-bit systems, since I've hardcoded the ELF interpreter path to ${stdenv.cc.libc}/lib/ld-linux-x86-64.so.2. Do I need to use a different ELF interpreter for 32-bit ELFs? If so, what path can it be found at? Alternatively, is choosing an appropriate ELF interpreter already handled somewhere else in nixpkgs, so I can just use something along the lines of ${stdenv.elfInterpreter} instead?
• This expression only works on NixOS at the moment. Getting it to work on Linux+Nix or macOS+Nix systems would require locating libGL.so.1 on that system. Is there a built in way to pick the right directory to add to LD_LIBRARY_PATH for this, or does this expression need to do the logic itself?
• As requested in [Package request] unigine #20547, I think it should be straightforward to package Unigine's other graphics benchmarks. Perhaps a single generic expression could be used for all of them.

[kierdavis]

CI failure seems to be unrelated. I'll try again when things are greener.

[abbradar]

Late to the party but of possible interest to later changes:

1. You can use "$(cat $NIX_CC/nix-support/dynamic-linker), for example: patchelf --interpreter "$(cat $NIX_CC/nix-support/dynamic-linker)";
2. If you do not add mesa_noglu to the library path it should work as is. On NixOS this is handled with LD_LIBRARY_PATH. On conventional Linux libGL.so.1 is found in standard system library paths. So is for macOS, I believe;

EDIT: Actually I'm not sure at all how things on macOS are, but they don't have libGL.so and don't even use ELF. I don't think you can make your expression compatible with macOS without a completely different version.