Auto merge of #5878 - bundler:seg-bundler-binstubs, r=indirect

Add a special bundler binstub that ensures the correct version is activated

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

Closes rubygems/bundler#5876.

> Once Bundler 2 ships, each application will depend on a 1.x or 2.x version of Bundler.

We want to make sure that the user always has an _easy_ and _convenient_ way to activate the version of Bundler their app is depending upon.

### What was your diagnosis of the problem?

> One way to ensure that the application will always get the version of Bundler that it needs is with an application-and-bundler-version specific binstub. Let's make it so that bundle binstubs bundler will generate a binstub for the correct version of Bundler, and simply running bin/bundle will always provide the correct version of Bundler.

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

My fix implements a `bundle` binstub when `bundle binstubs bundler` is run

### Why did you choose this fix out of the possible options?

I chose this fix because it allows us to dynamically select the correct Bundler version (with appropriate overrides, such as `$BUNDLER_VERSION` and `bundle update --bundler`), both when invoking `bin/bundle`, and also when invoking other bundler-generated binstubs.