Dynamic builds seem to use cache, static ones don't

[AnyOldName3]

When running from scratch, both variants take about an hour and a half. Once a successful build has happened, the dynamic builds take about seven minutes, but the static ones still take over an hour and a half. I don't see any obvious reason why this is. It doesn't matter long-term when presumably we'll mostly just be changing the referenced vcpkg commit instead of rebuilding the same commit over and over, but it's annoying at the moment when we're trying to get the infrastructure working.

[AnyOldName3]

The description above isn't accurate - while repeated static jobs don't just take seven minutes, they do only take an hour, which is still less than a fresh job. They're clearly using the cache at least a bit.