Don't touch PATH, GEM_HOME and GEM_PATH when testing formula

Library/Homebrew/dev-cmd/test.rb

@@ -35,7 +35,7 @@ module Homebrew
   def test
     args = test_args.parse
 
-    Homebrew.install_bundler_gems!
+    Homebrew.install_bundler_gems!(setup_path: false)
 
     require "formula_assertions"
     require "formula_free_port"

Library/Homebrew/utils/gems.rb

@@ -125,7 +125,11 @@ module Homebrew
     )
   end
 
-  def install_bundler_gems!(only_warn_on_failure: false)
+  def install_bundler_gems!(only_warn_on_failure: false, setup_path: true)
+    old_path = ENV["PATH"]
+    old_gem_path = ENV["GEM_PATH"]
+    old_gem_home = ENV["GEM_HOME"]
+
     install_bundler!
 
     ENV["BUNDLE_GEMFILE"] = File.join(ENV.fetch("HOMEBREW_LIBRARY"), "Homebrew", "Gemfile")
@@ -152,5 +156,12 @@ module Homebrew
     end
 
     setup_gem_environment!
+  ensure
+    unless setup_path
+      # Reset the paths. We need to have at least temporarily changed them while invoking `bundle`.
+      ENV["PATH"] = old_path
+      ENV["GEM_PATH"] = old_gem_path
+      ENV["GEM_HOME"] = old_gem_home
+    end
   end
 end

Library/Homebrew/utils/gems.rbi

@@ -22,6 +22,6 @@ module Homebrew
   sig { void }
   def install_bundler!; end
 
-  sig { params(only_warn_on_failure: T::Boolean).void }
+  sig { params(only_warn_on_failure: T::Boolean, setup_path: T::Boolean).void }
-  def install_bundler_gems!(only_warn_on_failure: false); end
+  def install_bundler_gems!(only_warn_on_failure: false, setup_path: false); end
 end