Correctly re-install extensions when running pristine for a git source

[wagenet]

Unfortunately, I was unable to figure out how to prevent it from reloading
the built extension from the global cache in 2.0. That means this is 1.16
only for now. I will investigate more.

Resolves #6294

[ghost]

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[wagenet]

@colby-swandale I agree, but as you can see from my comment, I was unable to get it to work on master due to my inability to clear the entry from the global cache.

[wagenet]

@colby-swandale the issue I'm seeing is that for git gems, instead or actually rebuilding, we're just loading from bundler_extension_cache_path.

https://github.com/bundler/bundler/blob/master/lib/bundler/rubygems_gem_installer.rb#L22

I believe the solution would either be to have the force option be true or to delete the extension_cache entry. Do you have a suggestion on which is preferable?

[wagenet]

Reopened in #6305 with a potential solution.