How to select the build to copy artifacts from, such as latest successful
or stable build, or latest "keep forever" build. Other plugins may provide
additional selections.
The build number of the selected build will be recorded in the environment
for later build steps to reference. The name of the environment variable
is COPYARTIFACT_BUILD_NUMBER_ with the source project name appended
(in all uppercase, and characters other than A-Z replaced with underscore).