Update CI workflow to use 'main' branch and cache v3 #57

Job	Run time
Julia 1 - macOS-latest - x64 - push	2m 22s
Julia 1.6 - macOS-latest - x64 - push	1m 47s
Julia nightly - macOS-latest - x64 - push	1m 38s
Julia nightly - ubuntu-latest - x64 - push	59s
Julia 1 - ubuntu-latest - x64 - push	1m 10s
Julia nightly - windows-latest - x64 - push	21m 21s
Julia 1.6 - ubuntu-latest - x64 - push	56s
	30m 13s

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