fail to load logstash/vendor/jruby/lib/ruby/stdlib/bundler in unit test

[kaisecheng]

All plugin unit tests failed in 8.6.0-snapshot but work with v8.5.2 due the removal of "vendor/jruby/bin/bundle*" in #14667

Build scan:
https://app.travis-ci.com/github/logstash-plugins/logstash-output-elasticsearch/jobs/590240695

Repro line:

Reproduces locally?:
yes

Applicable branches:
8.6.0-snapshot

Failure history:

Failure excerpt:

logstash_1_7ed7443a9cc5 | jruby 9.3.8.0 (2.6.8) 2022-09-13 98d69c9461 OpenJDK 64-Bit Server VM 17.0.5+8 on 17.0.5+8 +indy +jit [x86_64-linux]
logstash_1_7ed7443a9cc5 | LoadError: no such file to load -- /usr/share/logstash/vendor/jruby/lib/ruby/stdlib/bundler
logstash_1_7ed7443a9cc5 |            require at org/jruby/RubyKernel.java:1017
logstash_1_7ed7443a9cc5 |   require_relative at org/jruby/RubyKernel.java:1045
logstash_1_7ed7443a9cc5 |             <main> at /usr/share/logstash/vendor/jruby/lib/ruby/stdlib/bundler/setup.rb:6
logstash_1_7ed7443a9cc5 |            require at org/jruby/RubyKernel.java:1017
logstash_1_7ed7443a9cc5 |            require at /usr/share/logstash/vendor/jruby/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:85
ci_logstash_1_7ed7443a9cc5 exited with code 1