Warning: a recent release failed

[failure-checker[bot]]

The following release PRs may have failed:

• #550 - The release job failed -- check the build log.
• #548 - The release job failed -- check the build log.

[bshaffer]

Investigating the build logs, it seems that composer is failing to install based on a missing gmp extension. We should find out which container the release job is run on, and make sure that extension is installed.

See https://github.com/googleapis/releasetool for where these jobs are defined