fix: Remove postUnpack step to preserve vendor dependencies

The postUnpack step was deleting deps directories which may have
been removing vendored dependencies. cleanSourceWith already handles
source filtering, so postUnpack is redundant and potentially harmful.

nix/build.nix

@@ -34,24 +34,6 @@ beamPackages.mixRelease {
       && baseName != ".nix-services-started";
   };
 
-  # Filter function to include necessary files and directories
-  # but exclude build artifacts
-  postUnpack = ''
-    # Remove build artifacts and temp directories from source
-    find $sourceRoot -type d \( \
-      -name "_build" -o \
-      -name "deps" -o \
-      -name ".elixir_ls" -o \
-      -name ".elixir-tools" -o \
-      -name "node_modules" -o \
-      -name "cover" -o \
-      -name ".git" \
-    \) -exec rm -rf {} + 2>/dev/null || true
-
-    # Remove dotfiles that shouldn't be in the build
-    find $sourceRoot -maxdepth 1 -type f -name ".*" ! -name ".formatter.exs" ! -name ".credo.exs" -delete 2>/dev/null || true
-  '';
-
   # Mix environment
   mixEnv = "prod";