virtualgl-lib: fix broken startup script (#49185)

Replace /bin/sh with bash in vglrun.
'exec' fails with /bin/sh

pkgs/tools/X11/virtualgl/lib.nix

@@ -19,6 +19,12 @@ stdenv.mkDerivation rec {
 
   enableParallelBuilding = true;
 
+  postPatch = ''
+    # script calls exec, which fails with plain sh
+    substituteInPlace ./server/vglrun.in \
+      --replace '#!/bin/sh' '#!/usr/bin/env bash'
+  '';
+
   meta = with stdenv.lib; {
     homepage = http://www.virtualgl.org/;
     description = "X11 GL rendering in a remote computer with full 3D hw acceleration";