[linux] set default data directory location to '/var/lib/boinc'. #4777

Job	Run time
libs-build	1m 53s
apps-build-release	1m 34s
client-build	2m 20s
apps-build	2m 20s
apps-x86-build-release	1m 32s
apps-arm64-build-release	6m 51s
libs-vcpkg-build	2m 4s
client-vcpkg-build	2m 51s
apps-vcpkg-build	3m 12s
libs-cmake-build	1m 55s
libs-arm64-build	2m 5s
apps-arm64-build	7m 49s
manager-with-webview-vcpkg-build	8m 35s
server-build	3m 48s
manager-with-webview-build	5m 36s
manager-without-webview-build	5m 32s
unit-test-build	3m 17s
integration-test-build	4m 53s
	1h 8m 7s

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