Running everything in `bundled_app` by default causes the `bundled_app`
helper to be used everytime, and that will create a scoped bundled_app
folder if it does not exist. That causes `bin/rake spec:deps` to create
an empty `tmp/2.1/bundled_app` folder which is a bit weird.

This commit changes specs to not switch to a (possibly empty)
bundled_app directory when not necessary (for example, when running
`gem` commands in order to setup test dependencies).

https://github.com/rubygems/rubygems/commit/4bf89c0705

Since `sys_exec` does not actually set it.

https://github.com/rubygems/rubygems/commit/abc0fd0599

Overriding the version constant feels too magic and creates a set of
problems. For example, Bundler will lock the simulated version, and that
can cause issues when the lockfile is used under an environment not
simulating Bundler 4 (it will try to auto-install and auto-switch to a
version that does not exist).

On top of that, it can only be configured with an ENV variable which is
not too flexible.

This commit takes a different approach of using a setting, which is
configurable through ENV or `bundle config`, and pass the simulated
version to `Bundler::FeatureFlag`. The real version is still the one set
by `VERSION`, but anything that `Bundler::FeatureFlag` controls will use
the logic of the "simulated version".

In particular, all feature flags and deprecation messages will respect
the simulated version, and this is exactly the set of functionality that
we want users to be able to easily try before releasing it.

https://github.com/rubygems/rubygems/commit/8129402193

https://github.com/rubygems/rubygems/commit/4c450eb05e

Since we no longer pass ruby CLI flags in our spec commands, we no
longer need the previous workaround and can get the realworld code
tested.

https://github.com/rubygems/rubygems/commit/fd92c855fb

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

https://github.com/rubygems/rubygems/commit/1257bd161e