Sidestep multipart upload failures on GCS

- Attempting to upload a file to Google Cloud Storage larger than 5 MB
  (the default multipart upload size) will return:
```
error running command: InvalidArgument: Invalid argument.
  status code: 400, request id:
```
- This PR avoids the issue by disabling multipart uploads when the GCS
  endpoint is used, didn't have time to investigate an actual fix
- Didn't add an integration test as it seemed like a lot of overhead to
  require a GCS account in CI just for this change, but let me know if
  you'd rather have integration coverage around this.

s3client.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"strings"
 	"time"
 
 	"github.com/aws/aws-sdk-go/aws"
@@ -152,6 +153,11 @@ func (client *s3client) BucketFileVersions(bucketName string, remotePath string)
 func (client *s3client) UploadFile(bucketName string, remotePath string, localPath string, options UploadFileOptions) (string, error) {
 	uploader := s3manager.NewUploader(client.session)
 
+	if client.isGCSHost() {
+		// GCS returns `InvalidArgument` on multipart uploads
+		uploader.MaxUploadParts = 1
+	}
+
 	stat, err := os.Stat(localPath)
 	if err != nil {
 		return "", err
@@ -411,3 +417,7 @@ func (client *s3client) newProgressBar(total int64) *pb.ProgressBar {
 
 	return progress.SetWidth(80)
 }
+
+func (client *s3client) isGCSHost() bool {
+	return (client.session.Config.Endpoint != nil && strings.Contains(*client.session.Config.Endpoint, "storage.googleapis.com"))
+}