fix nested calls to extendDerivation

if extendDerivation is called on something that already had extendDerivation
called on it (eg a mkDerivation result) the second call will set
outputUnspecified=true on every output by way of propagating attributes of the
full derivation to the individual outputs. this in turn causes buildEnv--and
thus nix-shell and environment.systemPackages--to install every output of such a
derivation even when only a specific output was requested, which renders the
point of multiple outputs moot. this happens in python modules (see #139756),
though it seems that tcl and possibly others should also be affected.

lib/customisation.nix

@@ -145,7 +145,8 @@ rec {
     let
       outputs = drv.outputs or [ "out" ];
 
-      commonAttrs = drv // (builtins.listToAttrs outputsList) //
+      commonAttrs = (removeAttrs drv [ "outputUnspecified" ]) //
+        (builtins.listToAttrs outputsList) //
         ({ all = map (x: x.value) outputsList; }) // passthru;
 
       outputToAttrListElement = outputName: