Retrieve logstash-core gem's path using inline bundler

Release notes

[rn:skip]

What does this PR do?

Avoid to fork a bundler process to extract logstash-core's path and use the same bundler that wraps the execution of the Rakefile itself.

Why is it important/What is the impact to the user?

All the Rake tasks must run with same JRuby distribution that launched the execution. Without this, it uses the system's Ruby.

Checklist

[x] My code follows the style guidelines of this project
[x] I have commented my code, particularly in hard-to-understand areas
~~[ ] I have made corresponding changes to the documentation~~
~~[ ] I have made corresponding change to the default configuration files (and/or docker env variables)~~
~~[ ] I have added tests that prove my fix is effective or that my feature works~~

Author's Checklist

[x] run with system's JRuby
[x] run with not the system's JRuby

How to test this PR locally

Verify that it works with Logstash JRuby distribution:

export LOGSTASH_PATH=/tmp/logstash/ && export LOGSTASH_SOURCE=1
/tmp/logstash/bin/ruby -S bundle install
/tmp/logstash/bin/ruby -S exec rake vendor

Related issues

Closes #159

Use cases

Screenshots

Logs

If not configured int his way, running /tmp/logstash/bin/ruby -S exec rake vendor would rise:

andrea:logstash-filter-date (main) % rm gradle.properties && /tmp/logstash/bin/ruby -S bundle exec rake vendor
Using system java: /Users/andrea/.sdkman/candidates/java/current/bin/java
Processing jvm.options file at `/tmp/logstash/config/jvm.options`
Using GEM_HOME=/tmp/logstash/vendor/bundle/jruby/3.1.0
Using GEM_PATH=/tmp/logstash/vendor/bundle/jruby/3.1.0
DEBUG: exec /tmp/logstash/vendor/jruby/bin/jruby -S bundle exec rake vendor

Gem::GemNotFoundException: can't find gem bundler (= 2.3.26) with executable bundle
       find_spec_for_exe at /Users/andrea/.rvm/rubies/jruby-9.4.2.0/lib/ruby/stdlib/rubygems.rb:265
                bin_path at /Users/andrea/.rvm/rubies/jruby-9.4.2.0/lib/ruby/stdlib/rubygems.rb:245
                bin_path at /tmp/logstash/vendor/jruby/lib/ruby/stdlib/bundler/rubygems_integration.rb:178
    set_bundle_variables at /tmp/logstash/vendor/jruby/lib/ruby/stdlib/bundler/shared_helpers.rb:282
  set_bundle_environment at /tmp/logstash/vendor/jruby/lib/ruby/stdlib/bundler/shared_helpers.rb:76
                   setup at /tmp/logstash/vendor/jruby/lib/ruby/stdlib/bundler/runtime.rb:20
                   setup at /tmp/logstash/vendor/jruby/lib/ruby/stdlib/bundler.rb:161
                  <main> at /tmp/logstash/vendor/jruby/lib/ruby/stdlib/bundler/setup.rb:20
              with_level at /tmp/logstash/vendor/jruby/lib/ruby/stdlib/bundler/ui/shell.rb:136
                 silence at /tmp/logstash/vendor/jruby/lib/ruby/stdlib/bundler/ui/shell.rb:88
                  <main> at /tmp/logstash/vendor/jruby/lib/ruby/stdlib/bundler/setup.rb:20
                 require at org/jruby/RubyKernel.java:1057
                 require at /Users/andrea/.rvm/rubies/jruby-9.4.2.0/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:85
-------------------> Wrote /Users/andrea/workspace/logstash_plugins/logstash-filter-date/gradle.properties
logstashCoreGemPath=

logstash-plugins / logstash-filter-date

Retrieve logstash-core gem's path using inline bundler #160