Bundler 2.1.4 requires to use GEM_HOME

This fixes a shared gems storage for GitLab and Gitaly.
Prior to `2.1.x` we would use `BUNDLE_PATH`, but since
the new release, the bundler stores gems in `/ruby/<version>`
subfolder. The `.bundle_path` does return subfolder instead
of `BUNDLE_PATH` which causes problems with re-using gems storage.

lib/tasks/gitlab/gitaly.rake

@@ -22,7 +22,7 @@ Usage: rake "gitlab:gitaly:install[/installation/dir,/storage/path]")
       if Rails.env.test?
         command.push(
           'BUNDLE_FLAGS=--no-deployment',
-          "BUNDLE_PATH=#{Bundler.bundle_path}")
+          "GEM_HOME=#{Bundler.bundle_path}")
       end
 
       storage_paths = { 'default' => args.storage_path }

spec/tasks/gitlab/gitaly_rake_spec.rb

@@ -89,7 +89,7 @@ RSpec.describe 'gitlab:gitaly namespace rake task' do
           let(:command) do
             %W[make
                BUNDLE_FLAGS=--no-deployment
-               BUNDLE_PATH=#{Bundler.bundle_path}]
+               GEM_HOME=#{Bundler.bundle_path}]
           end
 
           before do