Fix bundle plugin install detection of already installed plugins

[deivid-rodriguez]

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

If you install a version of a plugin, say diffend 0.2.49, and then the latest version, 0.2.50, bundler will incorrectly think that 0.2.50 is already installed, and skip installation.

$ bundle plugin install diffend --version 0.2.49
Fetching gem metadata from https://rubygems.org/.
Resolving dependencies...
Using bundler 2.3.0.dev
Installing diffend 0.2.49
Installed plugin diffend

$ bundle plugin install diffend
Fetching gem metadata from https://rubygems.org/.
Resolving dependencies...
Using bundler 2.3.0.dev
Using diffend 0.2.50

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

Plugins are regular gems, but they are installed to their own GEM_HOME. My fix is to set the proper GEM_HOME used for plugins, so that the detection of whether a specific version is already installed works in the same way it works for regular gems.

Fixes #4864 (comment).

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• Write tests for features and bug fixes
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