fix gotestsum arch checks

[shivnagarajan]

This makes the download for gotestsum work on the various arch cases that we require (arm64 and x86_64)

Co-authored-by: Aaron Brady aaron.brady@shopify.com

[pawandubey]

Aah, thank you. Much better than what I was trying to do 🙃

[insom]

🎩 on a 64bit Pi, just in case:

raspberrypi:~/Repo/ghostferry$ make test
go version go1.15.9 linux/arm64
ulimit -n 1024 && ./bin/gotestsum --format short-verbose ./test/go ./copydb/test ./sharding/test -count 1 -p 1 -failfast