nix: prune bun platform binaries from package

[ry2009]

• Fixed the prune step so it actually works in Nix builds (copied .bun tree is read-only): nix/opencode.nix:96 now chmod -R u+w "$bun_root" before the rm -rf.
• Fixed the Nix interpolation bug by escaping shell ${...} expansions (''${...}) in nix/opencode.nix:99, so nix build doesn’t fail to parse.
• Verified with a real nixos/nix Docker build for aarch64-linux: output went from ~4.2 GiB (baseline) down to 2.8 GiB.