improve cache keys #216

Job	Run time
🏗️ Docker image	1m 29s
🔍 Python code format (3.8)	4m 6s
🔍 Python Lint (3.8)	4m 15s
🔍 Python code format (3.9)	3m 51s
🔍 Python Lint (3.9)	4m 15s
🔍 Python Lint backend (3.8, pyserini)	10m 43s
🔍 Python Lint backend (3.8, pyterrier)	6m 16s
🔍 Python Lint backend (3.9, pyserini)	4m 48s
🧪 Python unit tests (3.8)	4m 26s
🏗️ Python wheel (3.8)	15s
🔍 Python Lint backend (3.9, pyterrier)	5m 40s
🧪 Python unit tests (3.9)	4m 12s
🏗️ Python wheel (3.9)	15s
🧪 Python integration tests (3.8, pyserini)	11m 23s
🧪 Python integration tests (3.8, pyterrier)	4m 52s
🧪 Python integration tests (3.9, pyserini)	5m 55s
🧪 Python integration tests (3.9, pyterrier)	4m 42s
🚀 Publish Docker image	0s
🚀 Create GitHub release	0s
🚀 Publish Python wheels	0s
	1h 21m 23s

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