Attempt to fix broken apt-get update in CircleCI.
Attempt to fix broken apt-get update in CircleCI.

We've been getting this error on the `purge-unstable-repo` job for the last
couple days:

```
Reading package lists... Done

E: Could not open file
/var/lib/apt/lists/deb.debian.org_debian_dists_stretch-backports_main_binary-amd64_Packages.diff_Index
- open (2: No such file or directory)
Exited with code 100
```

Adding a double `apt-get update` seems to fix the issue, q.v.:

https://github.com/CircleCI-Public/gcp-cli-orb/issues/5#issuecomment-467616061

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