Revert to 'ProtectSystem=strict' in boinc-client.service and make '/tmp' writable #5219

Job	Run time
libs-build	5m 37s
apps-build	10m 8s
libs-cmake-build	6m 55s
client-armv6-build	2m 53s
client-arm-build	3m 39s
client-arm64-build	3m 38s
client-x86-build	3m 9s
client-x86_64-build	2m 56s
manager-build (assembleRelease)	3m 58s
manager-build (assembleXiaomi_release)	3m 52s
manager-build (assembleArmv6_release)	5m 13s
manager-build (jacocoTestReportDebug)	5m 46s
	57m 44s

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