[rubygems/rubygems] Add newline to validate_platforms! message when platform is missing

When I run bundle install with BUNDLE_DEPLOYMENT=true in the environment
on a different platform than I usually do development, I get the
following output to the console (wrapped exactly as shown):

Your bundle only supports platforms ["x86_64-darwin-19"] but your local platform
is x86_64-linux. Add the current platform to the lockfile with `bundle lock
--add-platform x86_64-linux` and try again.

Because the way the message wraps, its not as simple as copying the
suggested command to the clipboard because it contains a newline:

    $ bundle lock
    Writing lockfile to [...]/Gemfile.lock
    $ --add-platform x86_64-linux

Adding a newline right before the command forces the command in the
error message to be on the same line, which facilitates copy-pasting the
command in the message.

https://github.com/rubygems/rubygems/commit/4cf6989b11

lib/bundler/definition.rb

@@ -433,7 +433,7 @@ module Bundler
 
       raise ProductionError, "Your bundle only supports platforms #{@platforms.map(&:to_s)} " \
         "but your local platform is #{Bundler.local_platform}. " \
-        "Add the current platform to the lockfile with `bundle lock --add-platform #{Bundler.local_platform}` and try again."
+        "Add the current platform to the lockfile with\n`bundle lock --add-platform #{Bundler.local_platform}` and try again."
     end
 
     def add_platform(platform)

spec/bundler/commands/install_spec.rb

@@ -856,7 +856,7 @@ RSpec.describe "bundle install with gem sources" do
 
       expect(err).to eq(
         "Your bundle only supports platforms [\"x86_64-darwin-19\"] but your local platform is x86_64-linux. " \
-        "Add the current platform to the lockfile with `bundle lock --add-platform x86_64-linux` and try again."
+        "Add the current platform to the lockfile with\n`bundle lock --add-platform x86_64-linux` and try again."
       )
     end
   end