internal: Treat cfg fetching failures as a warning

If the user doesn't have rustc on $PATH, rust-analyzer won't be able
to run `rustc --print cfg`. This isn't really an error, as
rust-analyzer can still proceed without it.

This is particularly noticeable when loading crates defined in a
rust-project.json. Until the configuration is loaded, the opened files
are briefly treated as detached files and users see this error.

Environments with rust-project.json generally have a sysroot and rustc
elsewhere, so the error confuses users.

crates/project-model/src/toolchain_info/rustc_cfg.rs

@@ -19,7 +19,7 @@ pub fn get(
     let rustc_cfgs = match rustc_cfgs {
         Ok(cfgs) => cfgs,
         Err(e) => {
-            tracing::error!(?e, "failed to get rustc cfgs");
+            tracing::warn!(?e, "failed to get rustc cfgs");
             return vec![];
         }
     };