munin: Use outPath for packages in makeFlags

With this change, evaluating the `makeFlags` attribute does not result in building the referenced packages.

This makes it possible for users to override `buildInputs` and remove inputs they don't want without having to also make a modified copy of the `makeFlags` attribute.

An example use case is to remove the `jre` dependency.

pkgs/servers/monitoring/munin/default.nix

@@ -96,10 +96,10 @@ stdenv.mkDerivation rec {
     PREFIX=$(out)
     DESTDIR=$(out)
     PERLLIB=$(out)/${perlPackages.perl.libPrefix}
-    PERL=${perlPackages.perl}/bin/perl
+    PERL=${perlPackages.perl.outPath}/bin/perl
-    PYTHON=${python}/bin/python
+    PYTHON=${python.outPath}/bin/python
-    RUBY=${ruby}/bin/ruby
+    RUBY=${ruby.outPath}/bin/ruby
-    JAVARUN=${jre}/bin/java
+    JAVARUN=${jre.outPath}/bin/java
     PLUGINUSER=munin
   '';