cicd: enable caching through Swatinem/rust-cache@v2 #6

Job	Run time
Release harper-ls - Windows-x86_64	4m 37s
Release harper-ls - macOS-x86_64	1m 5s
Release harper-ls - macOS-aarch64	1m 32s
Release harper-ls - Linux-x86_64-GNU	1m 48s
Release harper-ls - Linux-aarch64-GNU	2m 11s
Release harper-cli - Windows-x86_64	3m 23s
Release harper-cli - macOS-x86_64	1m 15s
Release harper-cli - macOS-aarch64	1m 37s
Release harper-cli - Linux-x86_64-GNU	1m 3s
Release harper-cli - Linux-aarch64-GNU	1m 23s
	19m 54s

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