fix(aws): use correct signing region when the endpoint is overridden

[nikpivkin]

Trivy cannot get website configuration for a bucket, because an incorrect region is used.

Steps to reproduce:

Replace defsec (fixed configuration for local bucket)

replace github.com/aquasecurity/defsec => github.com/nikpivkin/defsec v0.0.0-20230727120552-23eac5dc734b

Run localstack

docker run \
  --rm -it \
  -p 4566:4566 \
  -p 4510-4559:4510-4559 \
  localstack/localstack

Create a bucket:

aws s3api create-bucket --endpoint http://0.0.0.0:4566 --bucket test-bucket --profile localstack --no-cli-pager
{
    "Location": "/test-bucket"
}

Run Trivy:

...
2023-07-28T17:13:25.174+0600	DEBUG	[defsec] 13:25.174209000 aws-api.scanner.adapt.aws        Error getting website: operation error S3: GetBucketWebsite, exceeded maximum number of attempts, 3, https response error StatusCode: 500, RequestID: 7aa7a34c-3393-4130-bb9e-ffcbd95b7c7c, HostID: s9lzHYrFp76ZVxRcpX9+5cjAnEH2ROuNkd2BHfIa6UkFVdtjf5mKR3/eTPFvsiP/XV/VLi31234=, api error InternalError: exception while calling s3.GetBucketWebsite: 'custom-signing-region' is not a valid AWS region name for s3
...

[simar7]

Fixed via aquasecurity/defsec#1411