ENV based variable substitution in `pipelines.yml` causes pipeline to fail to start

[robbavey]

Logstash information:

Please include the following information:

1. Logstash version (e.g. bin/logstash --version)

8.16.0

3. Logstash installation source (e.g. built from source, with a package manager: DEB/RPM, expanded from tar or zip archive, docker)

Built from source

Description of the problem including expected versus actual behavior:

When using an environment variable to populate the list of elasticsearch hosts in a pipeline configured in pipelines.yml, the pipeline fails to start.
Note that the same pipeline definition works successfully when supplied using -f or -e

Steps to reproduce:

1. Set an environment variable with an elasticsearch host in it - eg export PRODUCTION_ES_HOSTS="https://test-es-es-http.e2e-mercury.svc:9200"
2. Add a pipeline in pipelines.yml that refers to this variable:

- pipeline.id: test
 config.string: 'input { exec { command => "uptime" interval => 10 } } output { elasticsearch {hosts => "${PRODUCTION_ES_HOSTS}"}}'

3. Run bin/logstash

Provide logs (if relevant):

The pipeline will fail, with the following error:

[2024-08-09T12:30:36,236][ERROR][logstash.agent           ] Failed to execute action {:action=>LogStash::PipelineAction::Create/pipeline_id:test, :exception=>"LogStash::ConfigurationError", :message=>"Expected one of [A-Za-z0-9_-], [ \\t\\r\\n], \"#\", \"{\", [A-Za-z0-9_], \"}\" at line 1, column 91 (byte 91) after output { elasticsearch {hosts => https", :backtrace=>["/Users/robbavey/code/logstash/logstash-core/lib/logstash/compiler.rb:32:in `compile_imperative'", "org/logstash/execution/AbstractPipelineExt.java:239:in `initialize'", "org/logstash/execution/AbstractPipelineExt.java:173:in `initialize'", "/Users/robbavey/code/logstash/logstash-core/lib/logstash/java_pipeline.rb:48:in `initialize'", "org/jruby/RubyClass.java:949:in `new'", "/Users/robbavey/code/logstash/logstash-core/lib/logstash/pipeline_action/create.rb:49:in `execute'", "/Users/robbavey/code/logstash/logstash-core/lib/logstash/agent.rb:386:in `block in converge_state'"]}

[mashhurs]

#16375 fixed the issue.