Do not override @orig_env

[nobu]

What was the end-user or developer problem that led to this PR?

Since the merger of b545daa (ruby/ruby@6454238), TestFileExhaustive#test_expand_path_for_existent_username has been failing.

What is your fix for the problem, implemented in this PR?

Save to a local variable, instead of the instance variable used in teardown.
It is used only in with_env method, an instance variable has too wide scope.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[nobu]

https://github.com/rubygems/rubygems/actions/runs/9082602807/job/24959357217#step:10:32

  Environment variable changed: TestGem#test_self_clear_paths : "GEM_PRIVATE_KEY_PASSPHRASE" added
  Environment variable changed: TestGem#test_self_clear_paths : "GEM_SPEC_CACHE" added
  Environment variable changed: TestGem#test_self_clear_paths : "HOME" changed : "/home/runner" -> "/tmp/rubytest.3fwilm/test_rubygems_20240514-14918-j581fc/userhome"
  Environment variable changed: TestGem#test_self_clear_paths : "RUBYGEMS_PREVENT_UPDATE_SUGGESTION" added

https://github.com/rubygems/rubygems/actions/runs/9082602807/job/24959357217#step:10:80

  Environment variable changed: TestCiDetector#test_ci_strings : "GEM_SPEC_CACHE" changed : "/tmp/rubytest.3fwilm/test_rubygems_20240514-14918-j581fc/spec_cache" -> "/tmp/rubytest.3fwilm/test_rubygems_20240514-14918-5g4w0l/spec_cache"
  Environment variable changed: TestCiDetector#test_ci_strings : "HOME" changed : "/tmp/rubytest.3fwilm/test_rubygems_20240514-14918-j581fc/userhome" -> "/tmp/rubytest.3fwilm/test_rubygems_20240514-14918-5g4w0l/userhome"
  Environment variable changed: TestCiDetector#test_ci? : "GEM_SPEC_CACHE" changed : "/tmp/rubytest.3fwilm/test_rubygems_20240514-14918-5g4w0l/spec_cache" -> "/tmp/rubytest.3fwilm/test_rubygems_20240514-14918-odx44d/spec_cache"
  Environment variable changed: TestCiDetector#test_ci? : "HOME" changed : "/tmp/rubytest.3fwilm/test_rubygems_20240514-14918-5g4w0l/userhome" -> "/tmp/rubytest.3fwilm/test_rubygems_20240514-14918-odx44d/userhome"

[deivid-rodriguez]

Thank you @nobu 🙏