lib: Add optionalDrvAttr to conditionally set drv attributes.

This allows for adding new, conditionally set, derivation attributes
to an existing derivation without changing any output paths in the
case where the condition is not met.

lib/derivations.nix

@@ -98,4 +98,30 @@ in
       # `lazyDerivation` caller knew a shortcut, be taken from there.
       meta = args.meta or checked.meta;
     } // passthru;
+
+  /* Conditionally set a derivation attribute.
+
+     Because `mkDerivation` sets `__ignoreNulls = true`, a derivation
+     attribute set to `null` will not impact the derivation output hash.
+     Thus, this function passes through its `value` argument if the `cond`
+     is `true`, but returns `null` if not.
+
+     Type: optionalDrvAttr :: Bool -> a -> a | Null
+
+     Example:
+       (stdenv.mkDerivation {
+         name = "foo";
+         x = optionalDrvAttr true 1;
+         y = optionalDrvAttr false 1;
+       }).drvPath == (stdenv.mkDerivation {
+         name = "foo";
+         x = 1;
+       }).drvPath
+       => true
+  */
+  optionalDrvAttr =
+    # Condition
+    cond:
+    # Attribute value
+    value: if cond then value else null;
 }