Correct project_root path for ProjectJson.

It was already the folder containing the rust-project.json file, not the file itself.

crates/rust-analyzer/src/reload.rs

@@ -249,7 +249,7 @@ impl GlobalState {
                     // Enable flychecks for json projects if a custom flycheck command was supplied
                     // in the workspace configuration.
                     match config {
-                        FlycheckConfig::CustomCommand { .. } => project.path(),
+                        FlycheckConfig::CustomCommand { .. } => Some(project.path()),
                         _ => None,
                     }
                 }