Fix CI error in JRuby

```
NameError:
  uninitialized constant Set
```

See <https://github.com/GrottoPress/envy.rb/actions/runs/9333465138/job/25690519165>.

lib/envy.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "yaml"
+require "set"
 
 require_relative "envy/version"
 require_relative "envy/error"
@@ -47,12 +48,12 @@ def set_perms(files, perm: nil)
   # rubocop:disable Metrics/MethodLength, Metrics/CyclomaticComplexity
   def set_envs(yaml, prev_key = "", force:)
     case yaml
-    when ::Hash
+    when Hash
       yaml.each do |key, value|
         env_key = "#{prev_key}_#{key.to_s.upcase}".sub(/\A_/, "")
         set_envs(value, env_key, force: force)
       end
-    when ::Array, ::Set
+    when Array, Set
       yaml.each_with_index do |value, index|
         env_key = "#{prev_key}_#{index}".sub(/\A_/, "")
         set_envs(value, env_key, force: force)