fix build.gradle so that it can be used for normal dev. builds

[rursprung]

the current version of build.gradle contains checks which prevent it from just being run (or even just imported in an IDE, really): https://github.com/aparo/opensearch-analysis-ik/blob/4ac5760ec8b99bf02cb8a679f964e3ffa96f5b6d/build.gradle#L76-L86

• GITHUB_TOKEN is only needed when doing a release and the file will also only be present after it has been built (which isn't the case when parsing the build.gradle in an IDE or just listing its tasks).
• the copy task seems to be wrong as it ignores currentVersion and just copies the file to itself again (i.e. a noop?). also, this mustn't just be done in the main part of the build.gradle, this must be in a task which is only run upon release

[rursprung]

if you'd port the project to use this template then this should work out of the box: https://github.com/opensearch-project/opensearch-plugin-template-java