[Fix #10569] Fix a false positive for Style/FetchEnvVar

Fixes #10569.

This PR fixes a false positive for `Style/FetchEnvVar` when using
the same value of `ENV` as `if` condition in the body.

The `ENV` var as a `if` condition is currently allowed.
So the `ENV` var already used in the condition will be accepted as well.

And the following (maybe) corner case is accepted in this PR.

```ruby
if ENV['foo']
  ENV.delete('foo')
  puts 'some code'
  puts ENV['foo']
  puts 'other code'
end
```

#10569 (comment)

changelog/fix_a_false_for_style_fetch_env_var.md

@@ -0,0 +1 @@
+* [#10569](https://github.com/rubocop/rubocop/issues/10569): Fix a false positive for `Style/FetchEnvVar` when using the same `ENV` var as `if` condition in the body. ([@koic][])

lib/rubocop/cop/style/fetch_env_var.rb

@@ -97,7 +97,9 @@ def allowed_var?(node)
 
         def used_as_flag?(node)
           return false if node.root?
-          return true if node.parent.if_type?
+
+          if_node = node.ancestors.find(&:if_type?)
+          return true if if_node&.condition == node
 
           node.parent.send_type? && (node.parent.prefix_bang? || node.parent.comparison_method?)
         end

spec/rubocop/cop/style/fetch_env_var_spec.rb

@@ -296,6 +296,23 @@
         end
       RUBY
     end
+
+    it 'registers no offenses when using the same `ENV` var as `if` condition in the body' do
+      expect_no_offenses(<<~RUBY)
+        if ENV['X']
+          puts ENV['X']
+        end
+      RUBY
+    end
+
+    it 'registers no offenses when using an `ENV` var that is different from `if` condition in the body' do
+      expect_offense(<<~RUBY)
+        if ENV['X']
+          puts ENV['Y']
+               ^^^^^^^^ Use `ENV.fetch('Y')` or `ENV.fetch('Y', nil)` instead of `ENV['Y']`.
+        end
+      RUBY
+    end
   end
 
   context 'when the env val is excluded from the inspection by the config' do