Abort unsupported build steps

If we don't see machine that supports a build step for 'max_unsupported_time' seconds, the step is aborted. The default is 0, which is appropriate for Hydra installations that don't provision missing machines dynamically.

(cherry picked from commit f5cdbfe21d930db43d3812c7d8e87746d6378ef9)

Created by  Eelco Dolstra  on March 26, 2020
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