Add SNAPSHOT postfix when CI executes the 'snapshot' workflow type

[andsel]

Release notes

[rn:skip]

What does this PR do?

Updates the gemspec processing for logstash-core to include -SNAPSHOT when WORKFLOW_TYPE environment variable is snapshot, which happens when CI builds snapshot artifacts.

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

When running Logstash, the version stage or qualifier must be immediately visible, for example:

% bin/logstash --version
Using system java: /Users/andrea/.sdkman/candidates/java/current/bin/java
logstash 9.0.0-SNAPSHOT

Checklist

• My code follows the style guidelines of this project
• [ ] 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

• [ ]

How to test this PR locally

Export WORKFLOW_TYPE to snapshot and verify that the started Logstash print the version with the -SNAPSHOT postfix.

export $WORFLOW_TYPE=snapshot
./gradlew clean installDefaultGems
bin/logstash --version

Related issues

• Relates Use --qualifier in release manager #16907

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarQube

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 0da6849

cc @andsel

[dliappis]

@andsel I kicked off a build of the exhaustive test suite using this PR just to be on the safe side: https://buildkite.com/elastic/logstash-exhaustive-tests-pipeline/builds/1144

[donoghuc]

Do we want the same addition in the plugin API? https://github.com/elastic/logstash/blob/main/logstash-core-plugin-api/logstash-core-plugin-api.gemspec

[jsvd]

There are several use cases that need to be tested but some don't seem to work:

1. tarball name contains SNAPSHOT twice 🔴
2. snapshot shows up in bin/logstash -V 🟢
3. snapshot does not showsup in bin/logstash --debug -V, just shows 9.0.0 🔴
4. snapshot shows up in HTTP api response 🟢 (there is a separate "snapshot" field in the response)
5. snapshot does not show up in logging entry at startup, only 9.0.0 🔴

Also I suggest using RELEASE as the environment variable to know if the build is a release or a snapshot. WORKFLOW_TYPE is used by the ci scripts.