fix: expand env forcibly and double-check if file exists, make proper Go module

[JerusJ]

Just a couple changes:

• Set project go.mod to match project repo (so it can be go install'd)
• Fix shell expansion in cli args, also check if missing
• Update AWS SDK
• Change errors to just log.Fatal so we don't have to return/format

NOTE: the error:

Error getting identity: operation error STS: GetCallerIdentity, get identity: get credentials: failed to refresh cached credentials, no EC2 IMDS role found, operation error ec2imds: GetMetadata, request canceled, context deadline exceeded

Is only called if all credential providers are called but cannot be found. Since the config is loaded via: config.WithSharedCredentialsFiles([]string{sessionArgs.credFile}), if that is not correct/exists, then this red-herring error message is displayed.