Respect addBuildEnvironmentEntries for Build-Jdk-Spec

Fixed Apache Maven JAR Plugin manifest generation so Build-Jdk-Spec follows addBuildEnvironmentEntries and can be omitted for reproducible builds.

apache/maven-jar-plugin · #544

Problem

Projects could disable build environment manifest entries, but Build-Jdk-Spec still appeared in generated JAR manifests. That leaked environment-dependent JDK metadata and violated reproducible-build expectations for projects that intentionally disabled those entries.

Approach

Passed archive.manifest.addBuildEnvironmentEntries through to Maven Archiver's Build-Jdk-Spec default-entry switch and added an integration fixture that verifies environment manifest entries are absent while normal entries remain intact.

Impact and scope

Improves Maven JAR reproducibility by honoring the existing opt-out switch for environment-derived manifest metadata.
Keeps Main-Class and normal manifest behavior unchanged while suppressing volatile build-environment entries.
Adds coverage for modular Java JAR generation so the behavior is protected in plugin integration tests.

Validation

Added MJAR-539 integration test project and Groovy verifier for manifest contents.
Verified the targeted invoker test, mvn verify, and the full Maven JAR Plugin integration-test profile.
PR was accepted into the Maven JAR Plugin 4.0.0-beta-2 milestone.