[ruby/rubygems] Allow non-zero exit status in bundle config gemfile unset spec

After 607648d5fc9 (`bundle config get` exits 1 when the value is unset),
the spec added in 4658d6bd78b raises in the bundle helper because the
new test invokes `config get gemfile` after unsetting it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hsbt

spec/commands/config_spec.rb

@@ -605,7 +605,7 @@
       bundle "config set gemfile foo/bar_gemfile"
 
       bundle "config unset gemfile"
-      bundle "config get gemfile"
+      bundle "config get gemfile", raise_on_error: false
 
       expect(out).to include("You have not configured a value for `gemfile`")
       expect(File.read(bundled_app(".bundle/config"))).not_to include("BUNDLE_GEMFILE")