Make OpenJDK release-critical

Currently there are no tests that depend on the JDK. Since we don't
want a release with a broken JDK, make it an explicit dependency of
the "tested" jobs.

nixos/release-combined.nix

@@ -90,6 +90,7 @@ in rec {
 
         nixpkgs.tarball
         (all nixpkgs.emacs)
+        (all nixpkgs.jdk)
       ];
   });
 

nixos/release-small.nix

@@ -87,7 +87,10 @@ in rec {
     };
     constituents =
       let all = x: map (system: x.${system}) supportedSystems; in
-      [ nixpkgs.tarball ] ++ lib.collect lib.isDerivation nixos;
+      [ nixpkgs.tarball
+        (all nixpkgs.jdk)
+      ]
+      ++ lib.collect lib.isDerivation nixos;
   });
 
 }