Rename environment variable to AWS_DEFAULT_REGION

This seems to be the correct name, at least the AWS cli uses it:
https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-envvars.html

cmd/restic/global.go

@@ -486,7 +486,7 @@ func parseConfig(loc location.Location, opts options.Options) (interface{}, erro
 		}
 
 		if cfg.Region == "" {
-			cfg.Region = os.Getenv("AWS_REGION")
+			cfg.Region = os.Getenv("AWS_DEFAULT_REGION")
 		}
 
 		if err := opts.Apply(loc.Scheme, &cfg); err != nil {