runtime-spec: update pids.limit handling to match new guidance #8866

Job	Run time
cross-i386	1m 42s
test (ubuntu-24.04, 1.25.x, rootless, -race)	11m 57s
fedora	21m 3s
test (ubuntu-24.04, 1.25.x, -race)	8m 51s
test (ubuntu-24.04-arm, 1.24.x, rootless)	8m 21s
test (ubuntu-24.04, 1.24.x, rootless)	10m 25s
test (ubuntu-24.04, 1.25.x, criu-dev)	8m 57s
test (ubuntu-24.04-arm, 1.24.x)	6m 26s
test (ubuntu-24.04, 1.25.x, -race, criu-dev)	9m 17s
test (ubuntu-24.04, 1.25.x)	7m 57s
test (ubuntu-24.04, 1.24.x)	7m 41s
test (ubuntu-24.04-arm, 1.25.x, -race, criu-dev)	7m 20s
test (ubuntu-24.04-arm, 1.25.x, rootless, -race)	8m 27s
test (ubuntu-24.04-arm, 1.25.x, -race)	8m 3s
test (ubuntu-24.04-arm, 1.25.x)	6m 21s
test (ubuntu-24.04-arm, 1.25.x, criu-dev)	7m 27s
test (ubuntu-24.04-arm, 1.25.x, rootless)	7m 58s
test (ubuntu-24.04, 1.25.x, rootless)	11m 32s
all-done	2s
	2h 39m 47s

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