tar -tf will detect compressed tars correctly.

`tar -tzf` will seemingly blindly filter with gzip which causes
single-file ZIPs to be accidentally understood as tgz files.

If we match such a tar we explode trying to untar as that call doesn't
pass the option to uncompress and correctly spots that the file is not a
tar.

Closes #984.

workflow/executor/executor.go

@@ -614,7 +614,7 @@ func (we *WorkflowExecutor) AddAnnotation(key, value string) error {
 
 // isTarball returns whether or not the file is a tarball
 func isTarball(filePath string) bool {
- cmd := exec.Command("tar", "-tzf", filePath)
+ cmd := exec.Command("tar", "-tf", filePath)
  log.Info(cmd.Args)
  err := cmd.Run()
  return err == nil