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
Fixes #8454

Please indicate you've done the following:

• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file (make new-changelog) or comment /kind changelog-not-required on this PR.
• Updated the corresponding documentation in site/content/docs/main.

[reasonerjt]

By checking the caller of this func:

velero/pkg/repository/provider/unified_repo.go

Line 487 in 479c21c

credValue, err := getS3Credentials(config)

I don't quite understand in unified_repo.go how it is possible to enable refreshing the token automatically by returning nil.

Could you please elaborate?

[kaovilai]

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.

emphasis mine.