Inconsistent non-zero exit on setmeta

Command
```
- name: gcr.io/cloud-builders/gsutil
args:
["-m", "setmeta", "-h", "Content-Encoding", "gs://our-bucket/**/*.png", "gs://our-bucket/**/*.ico"]

```

Output:
```
/ [104/105 objects] 99% Done
/ [105/105 objects] 100% Done
Step #5: Operation completed over 105 objects.
Finished Step #5
ERROR
ERROR: build step 5 "gcr.io/cloud-builders/gsutil" failed: step exited with non-zero status: 1
```

We haven't changed any part of this build process, but this issue started sporadically occurring on May 25th, and now is happening almost every run. Running the same command from the command line works correctly.

Start by reproducing the `gsutil setmeta` command shown in the issue, including the `-m` flag and both image patterns, in Cloud Build and from the command line. Compare the exit status when all 105 objects report completion; done means the Cloud Build step exits successfully without the sporadic non-zero status.

Written by the indexing model from the issue text.