[rubygems/rubygems] Change spec to do the right thing on bundler 3

On bundler 3, the `--deployment` flag doesn't exist, so the `bundle
install --deployment` command was failing silently and the spec was
verifying a different scenario.

Change the spec to work the same regardless of bundler's major version.

Also, from the spec description it was not apparently that a specific
case involving deployment mode was being tested, so I reworded it to
make it more apparent.

https://github.com/rubygems/rubygems/commit/3e33e2b927

spec/bundler/runtime/setup_spec.rb

@@ -279,13 +279,13 @@ RSpec.describe "Bundler.setup" do
     end
 
     context "an absolute path is not provided" do
-      it "uses BUNDLE_GEMFILE to locate the gemfile if present" do
+      it "uses BUNDLE_GEMFILE to locate the gemfile if present and doesn't fail in deployment mode" do
         gemfile <<-G
           source "#{file_uri_for(gem_repo1)}"
         G
 
         bundle "install"
-        bundle "install --deployment"
+        bundle "config set --local deployment true"
 
         ENV["BUNDLE_GEMFILE"] = "Gemfile"
         ruby <<-R