build(agd): more careful `stamp.sum` handling #30257

Job	Run time
pre_check / Check preconditions and cancel previous jobs	5s
test-docker-build	16m 26s
deployment-test	3m 4s
getting-started (link-cli/yarn)	4m 29s
getting-started (registry/yarn)	7m 42s
getting-started (registry/npm)	7m 45s
getting-started (registry/npx)	7m 50s
finalize-integration-result	4s
	47m 25s

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