Use rust-analyzer's flycheck as source of cargo diagnostics (#29779)

Follow-up of https://github.com/zed-industries/zed/pull/29706

Instead of doing `cargo check` manually, use rust-analyzer's flycheck:
at the cost of more sophisticated check command configuration, we keep
much less code in Zed, and get a proper progress report.

User-facing UI does not change except `diagnostics_fetch_command` and
`env` settings removed from the diagnostics settings.

Release Notes:

- N/A

crates/collab/src/rpc.rs

@@ -328,6 +328,9 @@ impl Server {
                 forward_read_only_project_request::<proto::LspExtSwitchSourceHeader>,
             )
             .add_request_handler(forward_read_only_project_request::<proto::LspExtGoToParentModule>)
+            .add_request_handler(forward_read_only_project_request::<proto::LspExtCancelFlycheck>)
+            .add_request_handler(forward_read_only_project_request::<proto::LspExtRunFlycheck>)
+            .add_request_handler(forward_read_only_project_request::<proto::LspExtClearFlycheck>)
             .add_request_handler(
                 forward_read_only_project_request::<proto::LanguageServerIdForName>,
             )