Mittigate network transients in CI examples

[phlax]

We have added caching of one sort or another to most of CI and this has reduced massively our exposure to network transients - esp flakey apt mirrors

The examples dont currently have much in the way of caching - and its more difficult to do there, mostly as the cache key would expire too frequently, but also as the cache would be potentially large

There are parts of it we can cache tho - eg the bazel directories used in the wasm build

We could also add auto retries on the apt calls

cf https://askubuntu.com/a/1107071

[phlax]

cc @ggreenway

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