build.plat: add `build_{{name}}.json` output #3754

Job	Run time
document	54s
smoke (amaranth-lang/amaranth-soc)	56s
smoke (amaranth-lang/amaranth-boards)	33s
smoke (amaranth-lang/amaranth-stdio)	58s
check-links	57s
test (3.9)	2m 9s
test (3.13)	2m 33s
test (3.12)	3m 11s
test (pypy-3.9)	4m 58s
test (pypy-3.10)	5m 2s
test (pypy-3.11)	4m 46s
test (3.14-dev)	2m 41s
test (3.11)	2m 17s
test (3.10)	2m 43s
publish-docs	7s
publish-docs-dev	0s
required	2s
publish-package	17s
publish-schemas	5s
publish-release	0s
	35m 9s

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