SyncData: use $ref to say where room files are #1508

Job	Run time
CI (ubuntu-22.04, 6.4, e2ee)	16s
CI (ubuntu-22.04, 6.4)	15s
CI (ubuntu-22.04, 6.4, GCC, e2ee)	14s
CI (ubuntu-22.04, 6.4, GCC)	12s
CI (ubuntu-22.04, 5.15, e2ee)	16s
CI (macos-11, 6.4, e2ee)	6m 52s
CI (macos-11, 6.4)	3m 32s
CI (macos-11, 5.15, e2ee)	6m 28s
CI (windows-2019, 6.4, e2ee)	5m 3s
CI (windows-2019, 6.4)	2m 34s
CI (windows-2019, 5.15, e2ee)	4m 25s
CI (ubuntu-22.04, 5.15, e2ee, update-api, codeql)	19s
CI (ubuntu-22.04, 5.15, GCC, update-api)	18s
	30m 44s

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