We often hit "Too Many Requests" errors when uploading the cache with
`actions/cache` because there's a limit of 10 GB every 5 minutes, and we can
easily go over it when we amend or merge several PRs in a short timespan.
This will make the CI artifacts less useful for debugging crashes but there's
no real way around this.
... on all platforms but MSVC, as it still has a number of unsolved warnings
in its `/Wall` level. Some of it might be valid, others might be overkill,
this needs further assessment and fixes. (We could also change the `extra`
level to `/W4` on MSVC if that's more meaningful.)
