sherpaTarCreator: fix scons installation
Recent versions of scons from the pip install are not compatible with the software versions in SLC6/rel21 anymore. This commit makes the scons installation within sherpaTarCreator more robust.
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Recent versions of scons from the pip install are not compatible with the software versions in SLC6/rel21 anymore. This commit makes the scons installation within sherpaTarCreator more robust.