Read BUNDLE_VERSION env var in BundlerVersionFinder

Bundler::Settings resolves the `version` setting from the BUNDLE_VERSION
environment variable, but Gem::BundlerVersionFinder only consulted the
.bundle/config file. As a result `BUNDLE_VERSION=system` was ignored at
activation time and a lockfile-pinned bundler installed globally won
out over the default gem.

#9534

Author: hsbt

lib/rubygems/bundler_version_finder.rb

@@ -70,6 +70,9 @@ def self.lockfile_contents
   private_class_method :lockfile_contents
 
   def self.bundle_config_version
+    env_version = ENV["BUNDLE_VERSION"]
+    return env_version if env_version && !env_version.empty?
+
     version = nil
 
     [bundler_local_config_file, bundler_global_config_file].each do |config_file|

test/rubygems/test_gem_bundler_version_finder.rb

@@ -104,6 +104,48 @@ def test_bundler_version_with_bundle_config_version
     end
   end
 
+  def test_bundler_version_with_bundle_version_env_system
+    ENV["BUNDLE_VERSION"] = "system"
+
+    bvf.stub(:lockfile_contents, "\n\nBUNDLED WITH\n   1.1.1.1\n") do
+      assert_nil bvf.bundler_version
+    end
+  end
+
+  def test_bundler_version_with_bundle_version_env_overrides_config
+    ENV["BUNDLE_VERSION"] = "2.3.4"
+
+    config_content = <<~CONFIG
+      BUNDLE_VERSION: "1.2.3"
+    CONFIG
+
+    Tempfile.create("bundle_config") do |f|
+      f.write(config_content)
+      f.flush
+
+      bvf.stub(:bundler_global_config_file, f.path) do
+        assert_equal v("2.3.4"), bvf.bundler_version
+      end
+    end
+  end
+
+  def test_bundler_version_with_empty_bundle_version_env
+    ENV["BUNDLE_VERSION"] = ""
+
+    config_content = <<~CONFIG
+      BUNDLE_VERSION: "1.2.3"
+    CONFIG
+
+    Tempfile.create("bundle_config") do |f|
+      f.write(config_content)
+      f.flush
+
+      bvf.stub(:bundler_global_config_file, f.path) do
+        assert_equal v("1.2.3"), bvf.bundler_version
+      end
+    end
+  end
+
   def test_bundler_version_with_bundle_config_non_existent_file
     bvf.stub(:bundler_global_config_file, "/non/existent/path") do
       assert_nil bvf.bundler_version