Filter derivation with preferLocalBuild

ci.nix

@@ -3,10 +3,30 @@
 
 with import <nixpkgs> { };
 
+with lib;
+
 let
   src = import ./src;
+
+  attrs = genAttrs
+    [ "aarch64" "i686" ]
+    (arch: (src { inherit arch; }) // { recurseForDerivations = true; });
+
+  isCacheable = p: !(p.preferLocalBuild or false);
+  shouldRecurseForDerivations = p: isAttrs p && p.recurseForDerivations or false;
+
+  flattenPkgs = s:
+    let
+      f = p:
+        if shouldRecurseForDerivations p then flattenPkgs p
+        else if isDerivation p then [ p ]
+        else [ ];
+    in
+      concatMap f (attrValues s);
+
+  cachePkgs = filter isCacheable (flattenPkgs attrs);
+
+  outputsOf = p: map (o: p.${o}) p.outputs;
 in
 
-lib.genAttrs
-  [ "aarch64" "i686" ]
-  (arch: (src { inherit arch; }) // { recurseForDerivations = true; })
+concatMap outputsOf cachePkgs