nixos/release: add i686-linux mesa to channel blockers

Apart from being still a useful package, this keeps a basic sanity check
for i686, now that we dropped the installation image, etc.
Our RFC mandates that i686 stdenv is a channel blocker:
https://github.com/NixOS/rfcs/blob/master/rfcs/0046-platform-support-tiers.md#tier-3-1

nixos/release-combined.nix

@@ -167,6 +167,7 @@ in rec {
         (onFullSupported "nixos.tests.xfce")
         (onFullSupported "nixpkgs.emacs")
         (onFullSupported "nixpkgs.jdk")
+        (onSystems ["x86_64-linux"] "nixpkgs.mesa_i686") # i686 sanity check + useful
         ["nixpkgs.tarball"]
 
         # Ensure that nixpkgs-check-by-name is available in all release channels and nixos-unstable,

pkgs/top-level/all-packages.nix

@@ -23991,6 +23991,7 @@ with pkgs;
     inherit (darwin.apple_sdk_11_0.frameworks) OpenGL;
     inherit (darwin.apple_sdk_11_0.libs) Xplugin;
   };
+  mesa_i686 = pkgsi686Linux.mesa; # make it build on Hydra
 
   mesa_glu =  callPackage ../development/libraries/mesa-glu {
     inherit (darwin.apple_sdk.frameworks) ApplicationServices;