fix(coderd): workspaceapps: update last_used_at when workspace app reports stats (#11603)

- Adds a new query BatchUpdateLastUsedAt
- Adds calls to BatchUpdateLastUsedAt in app stats handler upon flush
- Passes a stats flush channel to apptest setup scaffolding and updates unit tests to assert modifications to LastUsedAt.

coderd/database/queries/workspaces.sql

@@ -357,6 +357,14 @@ SET
 WHERE
 	id = $1;
 
+-- name: BatchUpdateWorkspaceLastUsedAt :exec
+UPDATE
+	workspaces
+SET
+	last_used_at = @last_used_at
+WHERE
+	id = ANY(@ids :: uuid[]);
+
 -- name: GetDeploymentWorkspaceStats :one
 WITH workspaces_with_jobs AS (
 	SELECT