Speed up sources.sh even more

[yosifkit]

by improving slow jq section in sources.sh (building upon the gains from #115)

Local speed improvement:

$ cp sources.json sources-copy.json
$ # before change
$ time ../meta-scripts/sources.sh --cache-file=sources-copy.json > sources.json

real    1m15.459s
user    1m14.891s
sys     0m0.324s
$ # after
$ time ../meta-scripts/sources.sh --cache-file=sources-copy.json > sources.json

real    0m15.600s
user    0m15.071s
sys     0m0.477s

$ # with `sources.json` being correctly generated and unchanged from the checkout.

After tianon's improvements and a reorder 😍

$ time ../meta-scripts/sources.sh --cache-file=sources-copy.json > sources.json

real    0m5.095s
user    0m4.570s
sys     0m0.277s

And on a Jenkins worker versus the ~7 minutes in #115 (pre-feedback changes): 🎉 🎊 🥳

$ time ../doi-meta-scripts/sources.sh --cache-file=sources-copy.json > sources-cached.json

real    0m44.134s
user    0m43.897s
sys     0m1.458s

[tianon]

Suggested change

	value: {
	($e.arches | keys[]): [$e.sourceId]
	}
	value: { ($e.arches | keys[]): [$e.sourceId] },

[tianon]

Suggested change

	value: { ($s.arches | keys[]): [$s.sourceId] }
	value: { ($s.arches | keys[]): [$s.sourceId] },

[tianon]

Maybe we should also add a comment that this happens pre-arches-merge so looping over arches twice is fine?