mesa_drivers: fix drirc install path

This will install drirc to the location where mesa expects to find it.

You can test with:

    LIBGL_DEBUG=1 glxgears

An error message will be printed if drirc can't be found.

pkgs/development/libraries/mesa/default.nix

@@ -160,7 +160,7 @@ let self = stdenv.mkDerivation {
   doCheck = false;
 
   installFlags = [
-    "sysconfdir=\${out}/etc"
+    "sysconfdir=\${drivers}/etc"
     "localstatedir=\${TMPDIR}"
     "vendorjsondir=\${out}/share/glvnd/egl_vendor.d"
   ];