pkg_diffs.py: Keep known_failures #566

Job	Run time
unittests	16s
sanity	26s
focal-x64, broken / build debs	2m 45s
bookworm-x64, good, verbose=bloom / build debs	8m 35s
focal-x64, broken, cont-on-err / build debs	4m 57s
jammy-arm64, good, timeout=4 / build debs	4m 11s
jammy-x64, good / build debs	5m 5s
noble-x64, good, verbose=true / build debs	9m 24s
bookworm squash / deploy	0s
jammy push / deploy	0s
	35m 39s

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