nixos/mysql: run as mysql user and group

As we don't need to setup data directories from ExecStartPre= scripts
anymore, which required root, but use systemd.tmpfiles.rules instead,
everything can be run as just the mysql user.

nixos/doc/manual/release-notes/rl-1909.xml

@@ -162,6 +162,17 @@
       which is linked to <literal>fr-toutesvariantes.{aff,dic}</literal>.
     </para>
   </listitem>
+  <listitem>
+    <para>
+      The <literal>mysql</literal> service now runs as <literal>mysql</literal>
+      user. Previously, systemd did execute it as root, and mysql dropped privileges
+      itself.
+      This includes <literal>ExecStartPre=</literal> and
+      <literal>ExecStartPost=</literal> phases.
+      To accomplish that, runtime and data directory setup was delegated to
+      RuntimeDirectory and tmpfiles.
+    </para>
+   </listitem>
   </itemizedlist>
  </section>
 </section>

nixos/modules/services/databases/mysql.nix

@@ -326,6 +326,8 @@ in
         '';
 
         serviceConfig = {
+          User = cfg.user;
+          Group = "mysql";
           Type = if hasNotify then "notify" else "simple";
           # /run/mysqld needs to be created in addition to pidDir, as they could point to different locations
           RuntimeDirectory = "mysqld";