Fix test_app_start_without_rackup on JVM rubies

truffleruby and jruby uses the childprocess with
CHILDPROCESS_POSIX_SPAWN, and when doing so, the environment variables
aren't cocered to strings until very late:
https://github.com/enkessler/childprocess/blob/v4.1.0/lib/childprocess/unix/posix_spawn_process.rb#L109-L130

On CRuby it seems symbol environment variables will overwrite any
existing (string) ones but that does not happen on JVM rubies (I've only
tested with TruffleRuby though, but JRuby builds also failed in CI)

test/integration_start_test.rb

@@ -9,7 +9,7 @@ def test_app_start_without_rackup
     app_file = File.join(__dir__, "integration", "simple_app.rb")
     gem_file = File.join(__dir__, "integration", "gemfile_without_rackup.rb")
     command = command_for(app_file)
-    env = { BUNDLE_GEMFILE: gem_file }
+    env = { "BUNDLE_GEMFILE" => gem_file }
 
     with_process(command: command, env: env) do |process, read_io|
       assert wait_for_output(read_io, /Sinatra could not start, the "rackup" gem was not found/)