Allow initial git.build.time from project.build.outputTimestamp to support reproducible builds

[kylixs]

Describe your idea

following Maven's guide https://maven.apache.org/guides/mini/guide-reproducible-builds.html
After add a project.build.outputTimestamp property to the project's pom.xml with the timestamp value, the output should be reproducible.

But there is no way to use project.build.outputTimestamp as the initial value of git.build.time,
it is always initialized to the current time of the system:

  // git-commit-id-plugin-core : pl.project13.core.cibuild.BuildServerDataProvider
  private void loadBuildVersionAndTimeData(@Nonnull Properties properties) {
    Supplier<String> buildTimeSupplier = () -> {
      Date buildDate = new Date();
      SimpleDateFormat smf = new SimpleDateFormat(dateFormat);
      if (dateFormatTimeZone != null) {
        smf.setTimeZone(TimeZone.getTimeZone(dateFormatTimeZone));
      }
      return smf.format(buildDate);
    };
    maybePut(properties, GitCommitPropertyConstant.BUILD_TIME, buildTimeSupplier);

[TheSnoozer]

Thanks for creating an issue here. Good point. Sounds like the plugin should respect that property (if available)