extra/gen_package_index_json.sh: fix JSON format #644

Job	Run time
Build and package cores	8m 18s
Split off zephyr_main	12s
Split off zephyr_contrib	13s
Split off zephyr	13s
Split off zephyr_unoq	12s
Test zephyr_main:niclasense	2m 39s
Test zephyr_main:nano33ble	2m 42s
Test zephyr_main:giga	2m 50s
Test zephyr_main:portentac33	2m 44s
Test zephyr_contrib:ek_ra8d1	2m 34s
Test zephyr_contrib:frdm_rw612	2m 41s
Test zephyr:unoq	2m 33s
Test zephyr_contrib:frdm_mcxn947	2m 30s
Test zephyr_main:opta	2m 45s
Test zephyr_main:portentah7	2m 42s
Publish core	17s
Collect logs	6s
Publish jsons	12s
verify-core	0s
cleanup-artifacts	3s
	36m 26s

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