Fix demo seed Killed: 9 error on macos (#2138)

We've been encountering a persistent issue when running the `hydra-node`
demo on macOS with Apple Silicon (aarch64-darwin). The seed-devnet
process was consistently failing with a `Killed: 9` error.

The root cause of this issue was related to how the hydra-node
executable was being linked on macOS. The dynamic linker (dyld) was
unable to find the required shared libraries at runtime, which caused
the operating system to terminate the process.

Initially, I suspected aa code signing issue (which apparently is a
common cause for these types of errors on aarch64-darwin, as googling
suggested). But in the end, the problem was with the library search
paths embedded in the executable.

### Solution

The fix involved modifying the Nix build process to set the runtime
search paths for its library dependencies correctly. This is done by
using the `install_name_tool` command in the `postFixup` phase of the
Nix derivation.

Now it all seems to work (tested on my Mac machine).

Author: vrom911

nix/hydra/packages.nix

@@ -30,7 +30,8 @@
           # stdenv as the original drv (important to determine targetPlatform).
           drv.stdenv.mkDerivation {
             name = "${exe}";
-            phases = [ "buildPhase" ];
+            buildInputs = pkgs.lib.optionals pkgs.stdenv.isDarwin [ pkgs.cctools ];
+            phases = [ "buildPhase" ] ++ pkgs.lib.optionals pkgs.stdenv.isDarwin [ "postFixup" ];
             buildPhase = ''
               set -e
 
@@ -46,6 +47,13 @@
               sed 's/${placeholder}/${rev}/' ${drv}/bin/${exe} > $out/bin/${exe}
               chmod +x $out/bin/${exe}
             '';
+            postFixup = pkgs.lib.optionalString pkgs.stdenv.isDarwin ''
+              install_name_tool -add_rpath ${pkgs.zlib}/lib $out/bin/${exe}
+              install_name_tool -add_rpath ${pkgs.lmdb}/lib $out/bin/${exe}
+              install_name_tool -add_rpath ${pkgs.libcxx}/lib $out/bin/${exe}
+              install_name_tool -add_rpath ${pkgs.libiconv}/lib $out/bin/${exe}
+              install_name_tool -add_rpath ${pkgs.libffi}/lib $out/bin/${exe}
+            '';
           };
 
         nativePkgs = hsPkgs;