fix: accumulate image manifests in oci_image_index #1507

Job	Run time
matrix-prep-os	1s
matrix-prep-bazelversion	2s
test (ubuntu-latest, 7.1.1, ., true)	3m 15s
test (ubuntu-latest, 7.1.1, ., false)	2m 11s
test (ubuntu-latest, 7.1.1, e2e/wasm, false)	1m 59s
test (ubuntu-latest, 7.1.1, e2e/smoke, true)	19s
test (ubuntu-latest, 7.1.1, e2e/smoke, false)	16s
test (ubuntu-latest, 7.1.1, e2e/assertion, true)	2m 14s
test (ubuntu-latest, 6.4.0, ., true)	3m 34s
test (ubuntu-latest, 6.4.0, ., false)	2m 34s
test (ubuntu-latest, 6.4.0, e2e/wasm, false)	33s
test (ubuntu-latest, 6.4.0, e2e/smoke, true)	18s
test (ubuntu-latest, 6.4.0, e2e/smoke, false)	18s
test (ubuntu-latest, 6.4.0, e2e/assertion, true)	2m 32s
	20m 6s

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