lib/customisation: reduce // merges in extendDerivation

Fold the 'all' attribute into the listToAttrs call alongside the outputs, so commonAttrs builds from drv // listToAttrs(...) // passthru // { drvPath; outPath; } instead of four separate merges with a standalone 'all' attrset.

Also convert remaining optionalAttrs to nullable attr names in extendDerivation, overrideDerivation, and makeOverridable.

Per-derivation // merges drop from 4 to 3.

lib/customisation.nix

@@ -401,7 +401,25 @@ rec {
     condition: passthru: drv:
     let
       commonAttrs =
-        drv // (listToAttrs outputsList) // { all = map (x: x.value) outputsList; } // passthru;
+        drv
+        // listToAttrs (
+          outputsList
+          ++ [
+            {
+              name = "all";
+              value = map (x: x.value) outputsList;
+            }
+          ]
+        )
+        // passthru
+        // {
+          drvPath =
+            assert condition;
+            drv.drvPath;
+          outPath =
+            assert condition;
+            drv.outPath;
+        };
 
       outputsList = map (outputName: {
         name = outputName;
@@ -423,15 +441,7 @@ rec {
         };
       }) (drv.outputs or [ "out" ]);
     in
-    commonAttrs
-    // {
-      drvPath =
-        assert condition;
-        drv.drvPath;
-      outPath =
-        assert condition;
-        drv.outPath;
-    };
+    commonAttrs;
 
   /**
     Strip a derivation of all non-essential attributes, returning