contrib/rpm: make "snapshot" for package version configurable via script
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.
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