Only set the project artifact if it is the actual JAR

[mensinda]

Currently, the cached target/classes compile output is marked as the main project artifact when restoring the cache. This causes the issues described in #449. This PR ensures that only a "real" / JAR is marked as the project artifact when the cache is restored.

fixes #449

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[mensinda]

I have downgraded the JDK requirements and removed the toolchains plugin for the testcase. The pipeline should now (hopefully) pass.

[mensinda]

The build failure in https://github.com/apache/maven-build-cache-extension/actions/runs/21949285620/job/63461052355?pr=453 looks unrelated:

Error:  Failed to execute goal com.diffplug.spotless:spotless-maven-plugin:3.1.0:check (default) on project maven-build-cache-extension: Execution default of goal com.diffplug.spotless:spotless-maven-plugin:3.1.0:check failed: Unable to resolve dependencies: Failed to collect dependencies at com.github.ekryd.sortpom:sortpom-sorter:jar:4.0.0: Failed to read artifact descriptor for com.github.ekryd.sortpom:sortpom-sorter:jar:4.0.0: The following artifacts could not be resolved: com.github.ekryd.sortpom:sortpom-sorter:pom:4.0.0 (absent): Could not transfer artifact com.github.ekryd.sortpom:sortpom-sorter:pom:4.0.0 from/to central (https://repo.maven.apache.org/maven2): HTTP connect timed out -> [Help 1]
org.apache.maven.lifecycle.LifecycleExecutionException: Failed to execute goal com.diffplug.spotless:spotless-maven-plugin:3.1.0:check (default) on project maven-build-cache-extension: Execution default of goal com.diffplug.spotless:spotless-maven-plugin:3.1.0:check failed: Unable to resolve dependencies

With a lot of HTTP connect timed out in the stack trace.