Keep the original PATH in the EnvUtil.find_executable test

Replacing PATH with only the build directory breaks the spawned
ruby.exe on MSYS2 UCRT64, where the Windows DLL loader needs PATH to
resolve libgmp-10.dll linked into the ruby DLL. Prepend the directory
instead so the build ruby is still found first.

ruby/ruby@9272524

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: hsbt

test/test_find_executable.rb

@@ -9,7 +9,7 @@ def test_find_executable_returns_command_and_arguments
     ruby = RbConfig.ruby
     command = File.basename(ruby, RbConfig::CONFIG["EXEEXT"])
     original_path = ENV["PATH"]
-    ENV["PATH"] = File.dirname(ruby)
+    ENV["PATH"] = File.dirname(ruby) + File::PATH_SEPARATOR + original_path
 
     found = EnvUtil.find_executable(command, "--version") do |output|
       output.start_with?("ruby ")