Will be used by CI trigger to name packages that are build during testing
of a github pull request with the corresponding pull request ID.

"build_clean.sh" now supports a command line option -s|--snapshot. But the
same paramter can also be set via $NM_BUILD_SNAPSHOT environment
variable. Using the environment variable is useful to support older versions
and new versions of "build_clean.sh", so that the script can just ignore the
snapshot setting if it doesn't understand it yet.