Support building using system isal via ISAL_INSTALL_PREFIX (#15) #46

Job	Run time
test-cross (x86_64-unknown-linux-musl)	2m 39s
test-native (macos-13)	3m 58s
test-wasm32-emscripten (static)	59s
test-cross (s390x-unknown-linux-gnu)	2m 39s
test-native (macos-14)	1m 42s
test-wasm32-emscripten (shared)	1m 18s
test-cross (aarch64-unknown-linux-musl)	2m 40s
test-native (windows-latest)	3m 51s
test-native (ubuntu-latest)	2m 27s
	22m 13s

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