feat: cache sources too #1693

Job	Run time
Build x86_64-unknown-linux-musl	7m 36s
Build aarch64-unknown-linux-musl	8m 10s
Build powerpc64le-unknown-linux-gnu	8m 18s
Build x86_64-apple-darwin	9m 37s
test (ubuntu-latest)	1m 37s
test (macos-latest)	2m 16s
Build aarch64-apple-darwin	8m 41s
test (windows-latest)	3m 44s
Build x86_64-pc-windows-gnu	15m 23s
Build x86_64-pc-windows-msvc	15m 15s
Post-process	1s
	1h 20m 38s

Provide feedback