waf: make PYTHONPATH extensible (#408825)

Wrapping waf with "--set" prevents other python packages in
nativeBuildInputs to be used in a wscript. Wrapping with "--prefix"
allows us to overcome this limitation.

pkgs/by-name/wa/waf/package.nix

···
     runHook preInstall
 
     install -D waf "$out"/bin/waf
-    wrapProgram "$out"/bin/waf --set PYTHONPATH "$out"/${python3.sitePackages}
+    wrapProgram "$out"/bin/waf --prefix PYTHONPATH : "$out"/${python3.sitePackages}
     mkdir -p "$out"/${python3.sitePackages}/
     cp -r waflib "$out"/${python3.sitePackages}/
     runHook postInstall