_ci_: Have apt-get wait until it can get a lock in packer builds

[ianconsolata]

Related Issues

I've been debugging issues with the packer builds for a bit, and this is the next PR in that line. The digital ocean build was previously failing because it couldn't get an apt-get lock, I think because of some ubuntu auto-update functionality.

Proposed Changes

This has the packer build wait for 3 seconds and then try to get the lock again.

Checklist

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