web server+client: /manager/probe_sync

This fixes the following scenario:

```console
agama # cat rust/agama-lib/share/examples/profile_tw_gnome.json
{
  "software": {
    "patterns": [
      "gnome"
    ]
  }
}
agama # systemctl restart agama
agama # agama config load < rust/agama-lib/share/examples/profile_gnome.json
Anyhow(Backend call failed with status 400 and text '{"error":"Agama service error: Failed to find these patterns: [\"gnome\"]"}')
agama # systemctl restart agama
agama # PROBE_SYNC=1 agama config load < rust/agama-lib/share/examples/profile_gnome.json
agama #
```

Asynchronous probing results in a race(?) and an error when performing the
second PUT:

```
Sep 03 11:20:06 2cf2b88a0524 agama-web-server[9357]: request: GET /api/software/config
Sep 03 11:20:06 2cf2b88a0524 agama-web-server[9357]: request: PUT /api/software/config
Sep 03 11:20:06 2cf2b88a0524 agama-web-server[9357]: request: POST /api/manager/probe
Sep 03 11:20:06 2cf2b88a0524 agama-web-server[9357]: request: PUT /api/software/config
```

rust/agama-lib/src/manager/http_client.rs

@@ -18,6 +18,15 @@ impl ManagerHTTPClient {
     pub async fn probe(&self) -> Result<(), ServiceError> {
         // BaseHTTPClient did not anticipate POST without request body
         let empty_body: Vec<u8> = vec![];
+        if let Ok(value) = std::env::var("PROBE_SYNC") {
+            return if value == "1" {
+                self.client
+                    .post_void("/manager/probe_sync", &empty_body)
+                    .await
+            } else {
+                Ok(())
+            };
+        }
         self.client.post_void("/manager/probe", &empty_body).await
     }
 }

rust/agama-server/src/manager/web.rs

@@ -91,6 +91,7 @@ pub async fn manager_service(dbus: zbus::Connection) -> Result<Router, ServiceEr
     let state = ManagerState { manager, dbus };
     Ok(Router::new()
         .route("/probe", post(probe_action))
+        .route("/probe_sync", post(probe_sync_action))
         .route("/install", post(install_action))
         .route("/finish", post(finish_action))
         .route("/installer", get(installer_status))
@@ -134,6 +135,20 @@ async fn probe_action<'a>(State(state): State<ManagerState<'a>>) -> Result<(), E
     Ok(())
 }
 
+/// Starts the probing process.
+#[utoipa::path(
+    post,
+    path = "/probe_sync",
+    context_path = "/api/manager",
+    responses(
+      (status = 200, description = "Probing done.")
+    )
+)]
+async fn probe_sync_action(State(state): State<ManagerState<'_>>) -> Result<(), Error> {
+    state.manager.probe().await?;
+    Ok(())
+}
+
 /// Starts the probing process.
 #[utoipa::path(
     post,