Previously, if a classifier was set and build-image was executed from
the command-line, the build will fail as the plugin was unable to find
the original jar. This is because it relies on the attached artifacts
of the project, and those are not set when package has run as part of
a forked lifecycle.
This commit makes sure that the backup file is found by convention
first, the same way it is done for the target file.
Closes gh-26721
