Prioritize path.system over path when it makes sense

If `path.system` is configured locally, and `path` is configured
globally, gems should be installed to `path.system`.

bundler/lib/bundler/settings.rb

@@ -195,14 +195,16 @@ def pretty_values_for(exposed_key)
 
     # for legacy reasons, in Bundler 2, we do not respect :disable_shared_gems
     def path
-      key  = key_for(:path)
-      path = ENV[key] || @global_config[key]
-      if path && !@temporary.key?(key) && !@local_config.key?(key)
-        return Path.new(path, false)
+      configs.each do |_level, settings|
+        path = value_for("path", settings)
+        path_system = value_for("path.system", settings)
+        disabled_shared_gems = value_for("disable_shared_gems", settings)
+        next if path.nil? && path_system.nil? && disabled_shared_gems.nil?
+        system_path = path_system || (disabled_shared_gems == false)
+        return Path.new(path, system_path)
       end
 
-      system_path = self["path.system"] || (self[:disable_shared_gems] == false)
-      Path.new(self[:path], system_path)
+      Path.new(nil, false)
     end
 
     Path = Struct.new(:explicit_path, :system_path) do

bundler/spec/install/path_spec.rb

@@ -19,6 +19,14 @@
       expect(the_bundle).to include_gems "rack 1.0.0"
     end
 
+    it "uses system gems with `path.system` configured with more priority than `path`" do
+      bundle "config --local path.system true"
+      bundle "config --global path vendor/bundle"
+      bundle :install
+      run "require 'rack'", :raise_on_error => false
+      expect(out).to include("FAIL")
+    end
+
     it "handles paths with regex characters in them" do
       dir = bundled_app("bun++dle")
       dir.mkpath