ci: Use organisation-level AWS secrets

This commit updates the CI workflows to use the `zephyrproject-rtos`
organisation-level AWS secrets instead of the repository-level secrets.

Using organisation-level secrets allows more centralised management of
the access keys used throughout the GitHub Actions CI infrastructure.

Note that the `AWS_*_ACCESS_KEY_ID` is now stored in plaintext as a
variable instead of a secret because it is equivalent to username and
needs to be identifiable for management and audit purposes.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>
(cherry picked from commit 8233f8ef)