Gate the MAKEFLAGS jobserver spec on RubyGems 4.1

Older system RubyGems writes the build log to the extension directory, so
under RGV=system the example read the new build_info path and hit ENOENT.
The `-j` suppression it verifies only exists with the jobserver support in
RubyGems 4.1, so gate it like its sibling examples.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: hsbt

spec/commands/install_spec.rb

@@ -1389,7 +1389,7 @@ def gem_make_out
       end
     end
 
-    it "doesn't pass down -j to make when MAKEFLAGS is set" do
+    it "doesn't pass down -j to make when MAKEFLAGS is set", rubygems: ">= 4.1.0.dev" do
       ENV["MAKEFLAGS"] = "-j1"
 
       install_gemfile(<<~G, env: { "BUNDLE_JOBS" => "8" }, raise_on_error: false)