signal-desktop: Fix "Failed to load GLES library: libGLESv2.so.2"

A new symlink is required to fix the following error:
[3744707:0100/000000.911609:ERROR:egl_util.cc(74)] Failed to load GLES library: libGLESv2.so.2: libGLESv2.so.2: cannot open shared object file: No such file or directory
zsh: segmentation fault (core dumped) signal-desktop --enable-features=UseOzonePlatform --ozone-platform=wayland

The GPU acceleration still fails (not sure if it worked before) but at least
"signal-desktop --enable-features=UseOzonePlatform --ozone-platform=wayland"
launches again (without "--disable-gpu"):
[40492:0115/184719.611780:ERROR:gpu_process_host.cc(968)] GPU process exited unexpectedly: exit_code=139
[40492:0115/184720.256775:ERROR:gpu_process_host.cc(968)] GPU process exited unexpectedly: exit_code=139
[40492:0115/184720.892093:ERROR:gpu_process_host.cc(968)] GPU process exited unexpectedly: exit_code=139
[40620:0115/184721.033949:ERROR:sandbox_linux.cc(376)] InitializeSandbox() called with multiple threads in process gpu-process.
[40620:0115/184721.069600:ERROR:gl_utils.cc(318)] [.RendererMainThread-0x227200113f00]GL Driver Message (OpenGL, Performance, GL_CLOSE_PATH_NV, High): GPU stall due to ReadPixels
[40620:0115/184721.133265:ERROR:gl_utils.cc(318)] [.RendererMainThread-0x227200113f00]GL Driver Message (OpenGL, Performance, GL_CLOSE_PATH_NV, High): GPU stall due to ReadPixels
[40620:0115/184721.158341:ERROR:gl_utils.cc(318)] [.RendererMainThread-0x227200113f00]GL Driver Message (OpenGL, Performance, GL_CLOSE_PATH_NV, High): GPU stall due to ReadPixels

(After three GPU process crashes Chromium should automatically fall back
to software rendering.)

Fix #155050 (it only fixes the crashes though, not the underlying
issue, but that's likely all we can do for the moment as other Linux
distributions are affected as well; Ozone/Wayland is just not stable yet)

pkgs/applications/networking/instant-messengers/signal-desktop/default.nix

@@ -113,6 +113,9 @@ in stdenv.mkDerivation rec {
     mkdir -p $out/bin
     ln -s $out/lib/Signal/signal-desktop $out/bin/signal-desktop
 
+    # Create required symlinks:
+    ln -s libGLESv2.so $out/lib/Signal/libGLESv2.so.2
+
     runHook postInstall
   '';