[rubygems/rubygems] Allow enabling "Bundler 3 mode" more easily

Currently to test Bundler 3 mode we have to actually edit the version
file to simulate we're running a future version. This is inconvenient.

Instead, allow passing an environment variable, `BUNDLER_3_MODE`, to set
the "working mode" Bundler should use.

This can now be set easily by end users to enable them to try out the
changes in the future version and give us feedback.

It's unclear how version auto-switching should work when this
environment variable is set, so the auto-switching feature will be
disabled in that case.

https://github.com/rubygems/rubygems/commit/4e92e9b209

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