Compute correctly the git-tree-sha1 of each artifact #193

Job	Run time
Julia 1.6 - ubuntu-latest - x64 - pull_request	2m 6s
Julia 1.6 - macos-latest - x64 - pull_request	1m 53s
Julia 1.6 - windows-latest - x64 - pull_request	5m 43s
Julia 1 - ubuntu-latest - x64 - pull_request	1m 49s
Julia 1 - macos-latest - x64 - pull_request	1m 47s
Julia 1 - windows-latest - x64 - pull_request	2m 5s
Julia 1 - macos-latest - arm64 - pull_request	1m 27s
Julia nightly - ubuntu-latest - x64 - pull_request	2m 43s
Julia nightly - macos-latest - x64 - pull_request	4m 9s
Julia nightly - windows-latest - x64 - pull_request	3m 25s
	27m 7s

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