s3: use different strategy to resolve s3 region

The API endpoint GetBucketLocation requires
top level permission.

If we do an authenticated head request to a bucket, the bucket location will be returned in the HTTP headers.

See aws/aws-cli#2431 (comment)

backend/s3/s3.go

@@ -2992,19 +2992,26 @@ func (f *Fs) NewObject(ctx context.Context, remote string) (fs.Object, error) {
 
 // Gets the bucket location
 func (f *Fs) getBucketLocation(ctx context.Context, bucket string) (string, error) {
-	req := s3.GetBucketLocationInput{
+	req, _ := f.c.HeadBucketRequest(&s3.HeadBucketInput{
 		Bucket: &bucket,
-	}
-	var resp *s3.GetBucketLocationOutput
-	var err error
-	err = f.pacer.Call(func() (bool, error) {
-		resp, err = f.c.GetBucketLocation(&req)
-		return f.shouldRetry(ctx, err)
 	})
-	if err != nil {
-		return "", err
+	req.SetContext(ctx)
+	req.Config.Credentials = credentials.AnonymousCredentials
+
+	// Disable HTTP redirects to prevent an invalid 301 from eating the response
+	// because Go's HTTP client will fail, and drop the response if an 301 is
+	// received without a location header. S3 will return a 301 without the
+	// location header for HeadObject API calls.
+	req.DisableFollowRedirects = true
+
+	// an error (HTTP 307, 403, ...) is expected here but we are interested in HTTP headers
+	_ = req.Send()
+
+	if region := req.HTTPResponse.Header.Get("x-amz-bucket-region"); len(region) != 0 {
+		return s3.NormalizeBucketLocation(aws.StringValue(&region)), nil
 	}
-	return s3.NormalizeBucketLocation(aws.StringValue(resp.LocationConstraint)), nil
+
+	return "", fmt.Errorf("missing 'x-amz-bucket-region' http header")
 }
 
 // Updates the region for the bucket by reading the region from the