nixos/engelsystem: don't rely on `mysql` alias
nixos/engelsystem: don't rely on `mysql` alias

Since 3edde6562e19698da69a499881e0a2e4f5a497a2, we can no longer use
aliases inside the test framework. This has the implication that we can
no longer use aliases in any NixOS modules used by the test framework as
well (which is good), but does mean we need to clean up any instances
where this is the case.

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