Add missing apt-get update in CI test job

[EliahKagan]

The CI test job's "Setup dependencies" step had apt-get install without having previously run apt-get update. This often does not work, and in a CI environment one should not typically expect it to work because package indexes in a virtual machine just provisioned from an image, if present at all, may be very outdated. But for some reason it had been working until recently, when breakages were observed, including in #1924 (though the breakage is not related to the changes in that PR).

This runs apt-get update before apt-get install in that CI job, as had already been done in the other CI jobs and as had likely always been intended. This should make the "Setup dependencies" step work again.

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The reason I'm doing this as a separate PR rather than by adding a commit to #1924 is that it simplifies testing the same thing both in this upstream repository and in my fork, which I think might prove valuable in case any further troubleshooting is needed. (Doing it as a separate PR probably also makes the history slightly clearer, since this change is independent of the rationale of #1924, but that's secondary.)

I plan to make sure the test job gets past the "Setup dependencies" step, and the enable auto-merge.

[EliahKagan]

As anticipated (and hoped), the "Setup dependencies" step in the full test job passed, both in this upstream repository on the pull_request trigger where the problem seemed like it might not always happen, and downstream in the branch on my fork on the push trigger where I know it was repeatedly occurring.