When manually scheduling an eval, force re-instantiation of store derivations

Without this, if (failed or aborted) derivations have been garbage-collected, there is no way to restart them, which is very annoying. Now we set a forceEval flag in the jobset to cause it to be re-evaluated even if none of the inputs have changed.

Created by  Eelco Dolstra  on October 24, 2016
CCIORPJ5SKP5EVW4FXNDS7U7MXD3ZK65XRVBWI5QBJGETMQRPZUQC
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