slapos-node package: better use of python3.7

This is a fixup of 99d9c776

This way of forcing python3.7 is better because the name of the part is
still "python3" hence in the package we have

/opt/slapos/parts/python3/bin/python3

instead of

/opt/slapos/parts/python3.7/bin/python3
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