Fix another state leak in the test suite

Reproducible with

```
bin/rspec \
  ./spec/bundler/ruby_version_spec.rb[1:1:8:5:3:1] \
  ./spec/install/gemfile/ruby_spec.rb[1:4] \
  --seed 48267
```

1 files changed, 8 insertions, 2 deletions

diff --git a/spec/bundler/ruby_version_spec.rb b/spec/bundler/ruby_version_spec.rb
index 46a1b2918b..3ac7d9ef3a 100644
--- a/spec/bundler/ruby_version_spec.rb
+++ b/spec/bundler/ruby_version_spec.rb
@@ -399,8 +399,14 @@ RSpec.describe "Bundler::RubyVersion and its subclasses" do
 
       let(:bundler_system_ruby_version) { subject }
 
-      before do
-        Bundler::RubyVersion.instance_variable_set("@ruby_version", nil)
+      around do |example|
+        begin
+          old_ruby_version = Bundler::RubyVersion.instance_variable_get("@ruby_version")
+          Bundler::RubyVersion.instance_variable_set("@ruby_version", nil)
+          example.run
+        ensure
+          Bundler::RubyVersion.instance_variable_set("@ruby_version", old_ruby_version)
+        end
       end
 
       it "should return an instance of Bundler::RubyVersion" do