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perf(session): memoize project root lookups #4197

Job	Run time
build	9s
test (4/4)	1m 1s
test (1/4)	47s
static-analysis	38s
test (2/4)	54s
test (3/4)	43s
test (one-process pollution detector)	-1s
eslint	4s
prettier	3s
typecheck	3s
test (src)	3s
build-and-test	3s
	4m 27s