xorg-server: fix runtime errors

- add "bindnow" to `hardeningDisable`

Previously was there for 10 years:
https://github.com/NixOS/nixpkgs/commit/f519a255a56e7c42d04d1beb666d685078cc7e18

Removed during refactor:
https://github.com/NixOS/nixpkgs/commit/f7764be128f09512e80b7a911728ae52089cb959

Fixes runtime errors in nixos tests using `xorg-server`:
```
xserver-wrapper[823]: (II) LoadModule: "modesetting"
xserver-wrapper[823]: (II) Loading
/nix/store/lrbawv4ccl0x12nwrmbbzpxmzjj1z5r6-xorg-server-21.1.20/lib/xorg/modules/drivers/modesetting_drv.so
xserver-wrapper[823]: (EE) Failed to load
/nix/store/lrbawv4ccl0x12nwrmbbzpxmzjj1z5r6-xorg-server-21.1.20/lib/xorg/modules/drivers/modesetting_drv.so:
/nix/store/lrbawv4ccl0x12nwrmbbzpxmzjj1z5r6-xorg-server-21.1.20/lib/xorg/modules/drivers/modesetting_drv.so:
undefined symbol: gbm_bo_get_plane_count
xserver-wrapper[823]: (EE) Failed to load module "modesetting" (loader failed, 0)
xserver-wrapper[823]: (EE) No drivers available.
```

pkgs/by-name/xo/xorg-server/package.nix

@@ -151,6 +151,14 @@ stdenv.mkDerivation (finalAttrs: {
     mesa
   ];
 
+  hardeningDisable = [
+    # Removing this breaks GLAMOR due to failing to find libgbm symbols at runtime:
+    # Failed to load /...modesetting_drv.so: /...modesetting_drv.so: undefined symbol: gbm_bo_get_plane_count
+    # This is likely some load order weirdness, but we don't have a better solution for now.
+    # FIXME: fix this properly.
+    "bindnow"
+  ];
+
   propagatedBuildInputs = [
     dbus
     libepoxy