Normalize nixexpr{input,path} from builds to jobsetevals.

Duplicating this data on every record of the builds table cost approximately 4G of duplication.

Note that the database migration included took about 4h45m on an untuned server which uses very slow rotational disks in a RAID5 setup, with not a lot of RAM. I imagine in production it might take an hour or two, but not 4. If this should become a chunked migration, I can do that.

Note: Because of the question about chunked migrations, I have NOT YET tested this migration thoroughly enough for merge.

Created by  Graham Christensen  on January 22, 2021
MB5INTIQWM5VXEEWFNX46SWVNTORXWZGKZDOYZEORQ5T5ZM7XI2QC
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