testing/minio-client: upgrade to 0.20190920

Significant changes:
- Now that we are on go 1.13, enabling go111modules isn't needed
- With full go111modules support GOPATH is mostly there for caching
1 job for !70 with minio-client in 19 seconds
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failed #735
x86_64 docker-alpine allowed to fail
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00:00:19

 
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