pkg/alertmanager: Configure state directory

AlertManager is a stateful service, and as such needs a writable
location for its data.  Using the `StateDirectory` service unit
directive, we can configure systemd to create this directory
automatically, if it does not already exist when the service starts.

In order for the service to be able to write to the state directory, it
must have a stable UID.  This means it cannot use the `DynamicUser`
service unit directive.

package/alertmanager/alertmanager.service

@@ -15,11 +15,13 @@ ExecStart=/usr/sbin/alertmanager \
 	--web.listen-address=${WEB_LISTEN_ADDRESS}
 ExecReload=/bin/kill -HUP $MAINPID
 Restart=on-failure
+User=alertmanager
+StateDirectory=alertmanager
+WorkingDirectory=/var/lib/alertmanager
 
 CapabilityBoundingSet=
 DeviceAllow=
 DevicePolicy=closed
-DynamicUser=yes
 LockPersonality=yes
 MemoryDenyWriteExecute=yes
 NoNewPrivileges=yes