Avoid modifying frozen string in check_schema_file

This was missed when the frozen string literal pragma was added to this
file because the string is only modified when running in the context of
a full Rails app, which wasn't covered by the test suite.

activerecord/lib/active_record/tasks/database_tasks.rb

@@ -261,7 +261,7 @@ module ActiveRecord
 
       def check_schema_file(filename)
         unless File.exist?(filename)
-          message = %{#{filename} doesn't exist yet. Run `rails db:migrate` to create it, then try again.}
+          message = %{#{filename} doesn't exist yet. Run `rails db:migrate` to create it, then try again.}.dup
           message << %{ If you do not intend to use a database, you should instead alter #{Rails.root}/config/application.rb to limit the frameworks that will be loaded.} if defined?(::Rails.root)
           Kernel.abort message
         end

railties/test/application/rake/dbs_test.rb

@@ -259,6 +259,13 @@ module ApplicationTests
         end
       end
 
+      test "db:schema:load fails if schema.rb doesn't exist yet" do
+        Dir.chdir(app_path) do
+          stderr_output = capture(:stderr) { `bin/rails db:schema:load` }
+          assert_match /Run `rails db:migrate` to create it/, stderr_output
+        end
+      end
+
       def db_test_load_structure
         Dir.chdir(app_path) do
           `bin/rails generate model book title:string;