fix: assume IAM role before running cloudposse/github-action-atmos-get-setting

[goruha]

what

• assume IAM role before running cloudposse/github-action-atmos-get-setting

why

As of atmos 1.86.2, when atmos.Component began actually retrieving the TF state, it broke cloudposse/github-action-atmos-affected-stacks which we resolved as part of this release of the aforementioned action. We just had the action assume the IAM role, and that was it. However in cases where this function is used, appropriate IAM credentials to also be a requirement for cloudposse/github-action-atmos-get-setting:

> Run cloudposse/github-action-atmos-get-setting@v1
template: all-atmos-sections:163:26: executing "all-atmos-sections" at <atmos.Component>: error calling Component: exit status 1

Error: error configuring S3 Backend: IAM Role (arn:aws:iam::xxxxxxxxxxxx:role/xxxx-core-gbl-root-tfstate) cannot be assumed.

There are a number of possible causes of this - the most common are:
  * The credentials used in order to assume the role are invalid
  * The credentials do not have appropriate permission to assume the role
  * The role ARN is not valid

Error: NoCredentialProviders: no valid providers in chain. Deprecated.
	For verbose messaging see aws.Config.CredentialsChainVerboseErrors

references

https://github.com/cloudposse/atmos/releases/tag/v1.86.2

[github-actions]

These changes were released in v2.5.0.