` join-release-testnet.sh` improvements

[rootulp]

Context

I'm trying to join Cosmos Hub testnet by using join-release-testnet.sh

Problems

1. The script isn't executable. To fix: chmod +x join-release-testnet.sh
2. The script automatically downloads the amd64 version when my machine is arm64. Consider downloading the binary that is applicable for the user's current platform. You can check the platform via uname -a and parse the output. For posterity, if you're hitting this, you'll see an error like:

root@rootulp-arabica:~/testnets/release# journalctl -fu gaiad.service
Aug 27 14:45:28 rootulp-arabica systemd[1]: Started Gaia service.
Aug 27 14:45:28 rootulp-arabica systemd[105242]: gaiad.service: Failed to execute /root/go/bin/gaiad: Exec format error
Aug 27 14:45:28 rootulp-arabica systemd[105242]: gaiad.service: Failed at step EXEC spawning /root/go/bin/gaiad: Exec format error
Aug 27 14:45:28 rootulp-arabica systemd[1]: gaiad.service: Main process exited, code=exited, status=203/EXEC
Aug 27 14:45:28 rootulp-arabica systemd[1]: gaiad.service: Failed with result 'exit-code'.

[dasanchez]

Hi @rootulp ! Thanks for the input, we'll add a note in the docs so people know that the URL in that block should be modified to suit the relevant architecture.