Revert "Adjust environment when mrb_exec_irep happened."

[dearblue]

This reverts commit 26e436e.

After investigation, it is possible to revert by commit e89cc9b.

The build configuration file used in the investigation is shown below.

MRuby::Build.new do |conf|
  toolchain :clang

  enable_debug
  enable_bintest
  enable_test

  cc.command = "clang18"
  linker.command = "clang18"

  [cc, cxx].each { |c| c.defines << "MRB_GC_STRESS" }
  [cc, cxx, linker].each { |cmd| cmd.flags << %w(-fsanitize=address) }

  gem github: "iij/mruby-dir"       # rev: "89dceefa1250fb1ae868d4cb52498e9e24293cd1"
  gem github: "iij/mruby-env"       # rev: "056ae324451ef16a50c7887e117f0ea30921b71b"
  gem github: "iij/mruby-errno"     # rev: "b4415207ff6ea62360619c89a1cff83259dc4db0"
  gem github: "iij/mruby-require"   # rev: "f0634d785e5cbb73cd7d118ee36deff499e4181e"
  gem github: "iij/mruby-tempfile"  # rev: "9b883438547020dae328e34c8a2fe736171cd0ab"
end

Since the rake command needs to be from the past, we used the Ruby 2.6 version.