Preserve gem path so that we use the same gemspreserve-gem-path

Without this patch, I would end up with:

```
An error occurred in a `before(:suite)` hook.
Failure/Error: raise "could not connect to gitaly at #{socket.inspect} after #{sleep_time} seconds"

RuntimeError:
  could not connect to gitaly at "tmp/tests/gitaly/gitaly.socket" after 10 seconds
```

Digging into it, it's because `scripts/gitaly-test-spawn` could not
spawn the process, because it cannot find the installed gems.

I personally installed all my gems under $HOME, namely with:

* `gem install rake --user-install` or:
* `bundle install --path ~/.gem`

The gems would be installed to `~/.gem/ruby/2.4.0/gems`, where
the version is Ruby ABI version.

Now we're changing $HOME, making RubyGems think that the gems
would be installed to `tmp/tests/ruby/2.4.0/gems` which is
apparently not the case.

In order to workaround this, we could preserve $GEM_PATH
populated by RubyGems, ignoring the default path based on $HOME.

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