[Fix #10557] Fix a false positive for Style/FetchEnvVar

Fixes #10557.

This PR fixes a false positive for `Style/FetchEnvVar`
when `ENV['key']` is a receiver of `||=`.

changelog/fix_a_false_positive_for_style_fetch_env_var.md

@@ -0,0 +1 @@
+* [#10557](https://github.com/rubocop/rubocop/issues/10557): Fix a false positive for `Style/FetchEnvVar` when `ENV['key']` is a receiver of `||=`. ([@koic][])

lib/rubocop/cop/style/fetch_env_var.rb

@@ -64,11 +64,14 @@ def message_chained_with_dot?(node)
           node.parent.send_type? && node.parent.children.first == node && node.parent.dot?
         end
 
-        # Allow if used as a flag (e.g., `if ENV['X']` or `!ENV['X']`) because
-        # it simply checks whether the variable is set.
-        # Also allow if receiving a message with dot syntax, e.g. `ENV['X'].nil?`.
+        # The following are allowed cases:
+        #
+        # - Used as a flag (e.g., `if ENV['X']` or `!ENV['X']`) because
+        #   it simply checks whether the variable is set.
+        # - Receiving a message with dot syntax, e.g. `ENV['X'].nil?`.
+        # - `ENV['key']` is a receiver of `||=`, e.g. `ENV['X'] ||= y`.
         def allowable_use?(node)
-          used_as_flag?(node) || message_chained_with_dot?(node)
+          used_as_flag?(node) || message_chained_with_dot?(node) || node.parent&.or_asgn_type?
         end
       end
     end

spec/rubocop/cop/style/fetch_env_var_spec.rb

@@ -85,6 +85,14 @@
     end
   end
 
+  context 'when the node is a receiver of `||=`' do
+    it 'does not register an offense with `||`' do
+      expect_no_offenses(<<~RUBY)
+        ENV['X'] ||= y
+      RUBY
+    end
+  end
+
   context 'when it is an argument of a method' do
     it 'registers an offense' do
       expect_offense(<<~RUBY)