When Chromium is using EGL, it dlopen's libGLESv2. Before this fix, this failed
with a not-found error. Fix it by adding libGL to the RUNPATH of the binary.

This problem does not have a visible impact on typical desktop system because
Chromium uses GLX by default. It only matters when Chromium is started with
--use-gl=egl (which makes it use OpenGL ES via EGL instead of desktop OpenGL
via GLX), and probably on certain embedded systems which only support OpenGL
ES.

With this fix, Chromium runs fine for me with --use-gl=egl and the NVidia
driver, and the about:gpu page indicates that it is indeed using OpenGL ES and
EGL.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.