Update `BUILDING.md` and `PUBLISHING-AWS.md` to mention need for aws creds

[jpmcb]

Issue number:

Closes #2327

Description of changes:

Update BUILDING.md and PUBLISHING-AWS.md to mention need for aws creds

Several commands referenced in the developer docs interact with AWS
services; AMI, EKS, etc. In order to successfully publish AMIs via the
quickstart, AWS creds must be setup. This patch updates both BUILDING.md
and PUBLISHING-AWS.md to reflect the need to have this setup.

Signed-off-by: John McBride <[email protected]>

Testing done:

N/a

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.

[jpmcb]

Fore-pushed to addressed @stmcginnis comments around grammar and formatting

[jpmcb]

Force-pushed to resolve @zmrow's comment on removing "usually" - great suggestion! 👏🏼

[jpmcb]

Force-pushed to address Ben's comment on wording in BUILDING.md