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