fix(#8173): Do not return static aws credentials if they can expire

[DZDomi]

Please add a summary of your change

In a previous commit (30728c2) for the function that provides AWS credentials, a crucial if was removed that was responsible for making sure callers of the function GetS3Credentials would not receive expired credentials.

Specifically the removal of the following code caused the problem:

if os.Getenv(awsRoleEnvVar) != "" {
	return nil, nil
}

By not returning nil, nil in any of the code in the specific function and forcing to return some credentials, even if they can expire (for example for IAM roles that are assumed via STS) all functions that would call this function would end up with invalid tokens after some period. This affects specifically long running backup/data mover jobs to cloud storage like S3. This PR addresses this issue, buy checking if the credentials received are able to expire and if yes, will return nil,nil and let the calling functions get their credentials via the default AWS credential provider chain.

Does your change fix a particular issue?

Fixes #8173

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