feat: use S3 buildcache

Part of !410 (closed), factorizing. This doesn't write to S3, just reads from S3.

assigned to @wdconinc

added 1 commit

• d83044a0 - fix: install python3-boto3 for S3 mirror

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added 1 commit

• b8b2cdaf - fix: use s3://eictest/EPIC/spack

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added 4 commits

• 4473aa57 - 1 commit from branch master
• 7001c303 - feat: use S3 buildcache
• 14e90156 - fix: install python3-boto3 for S3 mirror
• eecbc0ec - fix: use s3://eictest/EPIC/spack

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enabled an automatic merge when the pipeline for eecbc0ec succeeds

merged