Bump julia-actions/cache from 1 to 2 #27

Job	Run time
Documentation	1m 15s
Julia 1.6 - ubuntu-latest - x64	1m 31s
Julia 1.6 - macos-latest - x64	2m 33s
Julia 1.6 - windows-latest - x64	12m 20s
Julia 1 - ubuntu-latest - x64	1m 41s
Julia 1 - macos-latest - x64	3m 3s
Julia 1 - windows-latest - x64	3m 49s
Julia nightly - ubuntu-latest - x64	1m 52s
Julia nightly - macos-latest - x64	5m 37s
Julia nightly - windows-latest - x64	3m 8s
Julia 1 - ubuntu-latest - x86	2m 22s
	39m 11s

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