Sidestep multipart upload failures on GCS
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.

concourse/s3-resource,concourse/s3-resource