[bundler/bundler] Reuse `gem_command!` helper

https://github.com/bundler/bundler/commit/37d5dedffe

spec/bundler/commands/clean_spec.rb

@@ -361,8 +361,7 @@ RSpec.describe "bundle clean" do
       gem "rack"
     G
 
-    gem = ruby_core? ? ENV["BUNDLE_GEM"] : "gem"
-    sys_exec! "#{gem} list"
+    gem_command! :list
     expect(out).to include("rack (1.0.0)").and include("thin (1.0)")
   end
 
@@ -484,8 +483,7 @@ RSpec.describe "bundle clean" do
     end
     bundle! :update, :all => true
 
-    gem = ruby_core? ? ENV["BUNDLE_GEM"] : "gem"
-    sys_exec! "#{gem} list"
+    gem_command! :list
     expect(out).to include("foo (1.0.1, 1.0)")
   end
 
@@ -509,8 +507,7 @@ RSpec.describe "bundle clean" do
     bundle "clean --force"
 
     expect(out).to include("Removing foo (1.0)")
-    gem = ruby_core? ? ENV["BUNDLE_GEM"] : "gem"
-    sys_exec "#{gem} list"
+    gem_command! :list
     expect(out).not_to include("foo (1.0)")
     expect(out).to include("rack (1.0.0)")
   end
@@ -544,8 +541,7 @@ RSpec.describe "bundle clean" do
       expect(err).to include(system_gem_path.to_s)
       expect(err).to include("grant write permissions")
 
-      gem = ruby_core? ? ENV["BUNDLE_GEM"] : "gem"
-      sys_exec "#{gem} list"
+      gem_command! :list
       expect(out).to include("foo (1.0)")
       expect(out).to include("rack (1.0.0)")
     end