Makefile: Add the ability to override the AMI data file suffix

[zmrow]

Description of changes:

This adds an additional variable to the `Makefile` that can be used to
override the suffix of the file where AMI data is written during publish
operations.  It is currently set to `amis.json`, which is identical to
its previous hard-coded value.

Overriding this variable can be useful when a user is publishing to
multiple places with different `Infra.toml` files and wishes to write
AMI data to specifically named files.

Testing done:

• Created an additional Infra-test.toml with different regions than my personal Infra.toml
• Ran cargo make ami, cargo make ami-public, cargo make ami-private, cargo make grant-ami, cargo make revoke-ami, and cargo make ssm (with a personal prefix) without any options and observed the "default" *amis.json file being used and correct actions being taken
• Ran cargo make ami, cargo make ami-public, cargo make ami-private, cargo make grant-ami, cargo make revoke-ami, and cargo make ssm (with a personal prefix) with -e "PUBLISH_INFRA_CONFIG_PATH=Infra-test.toml" -e "AMI_DATA_FILE_SUFFIX=amis-test.json" and observed the *amis-test.json file being used and correct actions being taken.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.