[bundler/bundler] bundler/inline should always install gems to system path

As discussed in the P/R, when `BUNDLE_PATH` env is set Bundler should
still install gems to the system path. `GEM_HOME` can be used to provide
different location if needed.

The test is added to document expected behavior of `bundler/inline`.

ae419fd6f8

spec/bundler/runtime/inline_spec.rb

@@ -291,6 +291,19 @@ RSpec.describe "bundler/inline#gemfile" do
     expect(out).to eq "1.0.0"
   end
 
+  context "when BUNDLE_PATH is set" do
+    it "installs inline gems to the system path regardless" do
+      script <<-RUBY, :env => { "BUNDLE_PATH" => "./vendor/inline" }
+        gemfile(true) do
+          source "file://#{gem_repo1}"
+          gem "rack"
+        end
+      RUBY
+      expect(last_command).to be_success
+      expect(system_gem_path("gems/rack-1.0.0")).to exist
+    end
+  end
+
   it "skips platform warnings" do
     simulate_platform "ruby"