Allow to override environment variables for debug

Author: nobu

test/-ext-/bug_reporter/test_bug_reporter.rb

@@ -9,8 +9,6 @@ def yjit_enabled?
   end
 
   def test_bug_reporter_add
-    omit if ENV['RUBY_ON_BUG']
-
     description = RUBY_DESCRIPTION
     description = description.sub(/\+RJIT /, '') unless JITSupport.rjit_force_enabled?
     expected_stderr = [
@@ -27,6 +25,7 @@ def test_bug_reporter_add
     args = ["--disable-gems", "-r-test-/bug_reporter",
             "-C", tmpdir]
     args.push("--yjit") if yjit_enabled? # We want the printed description to match this process's RUBY_DESCRIPTION
+    args.unshift({"RUBY_ON_BUG" => nil})
     stdin = "#{no_core}register_sample_bug_reporter(12345); Process.kill :SEGV, $$"
     assert_in_out_err(args, stdin, [], expected_stderr, encoding: "ASCII-8BIT")
   ensure

test/ruby/test_rubyoptions.rb

@@ -789,11 +789,10 @@ module SEGVTest
   end
 
   def assert_segv(args, message=nil)
-    omit if ENV['RUBY_ON_BUG']
-
     # We want YJIT to be enabled in the subprocess if it's enabled for us
     # so that the Ruby description matches.
     args.unshift("--yjit") if self.class.yjit_enabled?
+    args.unshift({'RUBY_ON_BUG' => nil})
 
     test_stdin = ""
     opt = SEGVTest::ExecOptions.dup

tool/lib/envutil.rb

@@ -155,7 +155,7 @@ def invoke_ruby(args, stdin_data = "", capture_stdout = false, capture_stderr =
 
     # remain env
     %w(ASAN_OPTIONS RUBY_ON_BUG).each{|name|
-      child_env[name] = ENV[name] if ENV[name]
+      child_env = ENV[name] if !child_env.key?(name) and ENV.key?(name)
     }
 
     args = [args] if args.kind_of?(String)