Change ecsanywhere install script to point to amazon-ecs-agent repo

[Realmonia]

Summary

Change ecsanywhere install script to point to amazon-ecs-agent repo. This is a missing change when we move init to agent repo

before: https://github.com/aws/amazon-ecs-init/tree/master/scripts
after: https://github.com/aws/amazon-ecs-agent/tree/master/scripts

Implementation details

Testing

New tests cover the changes:
Manually run on an ec2 instance to verify the download and verify signature works.

##########################
# Trying to install ssm agent ...


##########################
# Trying to verify the signature of amazon-ssm-agent package ...

/bin/gpg
gpg: key 693ECA21: "SSM Agent <[email protected]>" not changed
gpg: key 56BAA549: "SSM Agent <[email protected]>" not changed
gpg: Total number processed: 2
gpg:              unchanged: 2
gpg: Signature made Wed 09 Aug 2023 06:34:52 PM UTC using RSA key ID 56BAA549
gpg: Good signature from "SSM Agent <[email protected]>"
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: 2BC7 C7C2 67BB D505 EAA4  91E6 DD81 A617 56BA A549
amazon-ssm-agent GPG verification passed. Install the amazon-ssm-agent.

Description for the changelog

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.