Combine measurement and stddev columns #1731

Job	Run time
benchmark-ractor (head)	6m 22s
benchmark-default (ruby)	14m 25s
benchmark-run-once	1m 40s
test (ruby)	54s
benchmark-default (truffleruby, --excludes=knucleotide,lobsters,ruby-lsp,shipit)	13m 7s
test (truffleruby)	1m 35s
test (head)	41s
benchmark-default (head)	13m 22s
benchmark-graph	43s
benchmark-ractor-only (head)	37s
all-tests	2s
	53m 28s

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