I still get:

❯ radiance

thread 'main' (2015441) panicked at /build/cargo-vendor-dir/xkbcommon-dl-0.4.2/src/x11.rs:59:28:
Library libxkbcommon-x11.so could not be loaded.
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::option::expect_failed
   3: xkbcommon_dl::x11::xkbcommon_x11_handle
   4: std::sync::poison::once::Once::call_once_force::{{closure}}
   5: std::sys::sync::once::futex::Once::call
   6: std::sync::once_lock::OnceLock<T>::initialize
   7: winit::platform_impl::linux::common::xkb::Context::from_x11_xkb
   8: winit::platform_impl::linux::EventLoop<T>::new
   9: radiance::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

Any ideas?

  buildInputs =
    [
      mpv-unwrapped
      vulkan-loader
      libxkbcommon
    ]
    ++ lib.optionals stdenv.isLinux [
      alsa-lib
      wayland

      # X11 / XCB stack (required by winit + xkbcommon-dl)
      xorg.libX11
      xorg.libXcursor
      xorg.libXi
      xorg.libXrandr
      xorg.libXinerama
      xorg.libXpresent
      xorg.libXfixes
      xorg.libXext
      # xorg.libXss
      xorg.libxcb
      # xorg.libX11-xcb
    ]
    ++ lib.optionals stdenv.isDarwin [
      darwin.apple_sdk.frameworks.AppKit
      darwin.apple_sdk.frameworks.CoreGraphics
      darwin.apple_sdk.frameworks.CoreServices
      darwin.apple_sdk.frameworks.Foundation
      darwin.apple_sdk.frameworks.Metal
      darwin.apple_sdk.frameworks.QuartzCore
    ];

I did:

  preFixup = lib.optionalString stdenv.hostPlatform.isLinux ''
    patchelf \
      --set-rpath --add-needed "${lib.makeLibraryPath buildInputs}:$out/lib" \
      $out/bin/radiance
  '';

And now I get:

radiance> Running phase: fixupPhase
radiance> patchelf: getting info about '/nix/store/5vwc8q3c7hn7s01qbpsicv4zjk3p2nci-mpv-0.41.0/lib:/nix/store/jzng5i2xwsdlf3h60a5s6qzr2808c9zc-vulkan-loader-1.4.335.0/lib:/nix/store/91mg839gbvgz1svzlh1jvz0pqvx7ylwh-libxkbcommon-1.11.0/lib:/nix/store/f0mf3rlv29ql40qx36kqd2jzabd1m0s0-alsa-lib-1.2.14/lib:/nix/store/7m6x6b4ljrkmkbhvbxdcacpraxj49xkj-wayland-1.24.0/lib:/nix/store/ii3ybky5dqjikcrw7vdnh1j76ssy0ycm-libx11-1.8.12/lib:/nix/store/bb5xxw11ndww7iivcmdpxga9n1da24vg-libxcursor-1.2.3/lib:/nix/store/x1f9a0qsj6a1y5nf178naagm2vbxnazc-libxi-1.8.2/lib:/nix/store/dyn0y5clf5b556yqwmj4841h43hz75p6-libxrandr-1.5.4/lib:/nix/store/0r6d7iw0q9wgxxj28zy87n1gjwvk0klp-libxinerama-1.1.5/lib:/nix/store/hlkf0rilmbwi8hvbvc9rswglmsc1mpq5-libxpresent-1.0.2/lib:/nix/store/2dac9gfiim278y2c6mxrklpzzfka7ra1-libxfixes-6.0.2/lib:/nix/store/zshby6nalhw4mvap0rr97hv042808c2k-libxext-1.3.6/lib:/nix/store/m0jzvlr5gl7zvdn0fr7c09mjwx65fx58-libxcb-1.17.0/lib:/nix/store/d3dgbv5q78fdcswpznc20ipnjdna4mcp-radiance-0.7.0/lib': No such file or directory