Prefer err to last_command.bundler_err

Because it takes into account the target stream for errors according to
bundler's version.

spec/lock/lockfile_spec.rb

@@ -183,7 +183,7 @@
                       "than the version that created the lockfile (9999999.1.0). " \
                       "We suggest you to upgrade to the version that created the " \
                       "lockfile by running `gem install bundler:9999999.1.0`."
-    expect(last_command.bundler_err).to include warning_message
+    expect(err).to include warning_message
 
     lockfile_should_be <<-G
       GEM
@@ -226,7 +226,7 @@
     G
 
     expect(last_command).to be_failure
-    expect(last_command.bundler_err).to include("You must use Bundler 9999999 or greater with this lockfile.")
+    expect(err).to include("You must use Bundler 9999999 or greater with this lockfile.")
   end
 
   it "shows a friendly error when running with a new bundler 2 lockfile" do
@@ -1421,7 +1421,7 @@ def set_lockfile_mtime_to_known_value
       gem "rack"
     G
 
-    expect(last_command.bundler_err).to match(/your Gemfile.lock contains merge conflicts/i)
-    expect(last_command.bundler_err).to match(/git checkout HEAD -- Gemfile.lock/i)
+    expect(err).to match(/your Gemfile.lock contains merge conflicts/i)
+    expect(err).to match(/git checkout HEAD -- Gemfile.lock/i)
   end
 end