Fix bundler installing gems for a different platform when running in frozen mode and current platform not in the lockfile

[deivid-rodriguez]

What was the end-user or developer problem that led to this PR?

Since bundler 2.2.1, bundler could end up installing dependencies not valid for the current platform.

What is your fix for the problem, implemented in this PR?

RUBY is no longer included as a platform in the lockfile, however it's still included in the list of rubygems platforms, so the check was not doing the right thing.

Instead, use our own logic.

Fixes #4166.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[deivid-rodriguez]

I noticed that this error can only happen in frozen mode, so I think it'd be worth mentioning that as the cause of the error. I'll push an update.

However, I'm not sure about remediation steps. It's somewhat tricky because bundle lock --add-platform x86_64-linux is not going to work in frozen mode (I wonder if a command explicitly meant to update the lockfile should override frozen mode, but that's another story). So we'll need to mention that the command needs to be run in a non frozen environment, or maybe mention bundle config unset frozen && bundle lock --add-platform x86_84-linux && bundle config set --local frozen true directly?

Any ideas, @indirect, @knu?

[deivid-rodriguez]

I'll merge this now, and we can further tweak it later.