Cache cross-signing and megolm keys locally #1385

Job	Run time
CI (ubuntu-22.04, 6.4, e2ee)	6m 32s
CI (ubuntu-22.04, 6.4)	1m 37s
CI (ubuntu-22.04, 6.4, GCC, e2ee)	12m 57s
CI (ubuntu-22.04, 6.4, GCC)	1m 31s
CI (ubuntu-22.04, 5.15, e2ee)	6m 58s
CI (macos-11, 6.4, e2ee)	9m 20s
CI (macos-11, 6.4)	2m 33s
CI (macos-11, 5.15, e2ee)	8m 38s
CI (windows-2019, 6.4, e2ee)	7m 10s
CI (windows-2019, 6.4)	1m 53s
CI (windows-2019, 5.15, e2ee)	5m 58s
CI (ubuntu-22.04, 5.15, e2ee, update-api, codeql)	12m 4s
CI (ubuntu-22.04, 5.15, GCC, update-api)	3m 0s
	1h 20m 11s

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