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/editor/src/actions.rs

@@ -249,7 +249,9 @@ actions!(
         ApplyDiffHunk,
         Backspace,
         Cancel,
+        CancelFlycheck,
         CancelLanguageServerWork,
+        ClearFlycheck,
         ConfirmRename,
         ConfirmCompletionInsert,
         ConfirmCompletionReplace,
@@ -372,6 +374,7 @@ actions!(
         RevertFile,
         ReloadFile,
         Rewrap,
+        RunFlycheck,
         ScrollCursorBottom,
         ScrollCursorCenter,
         ScrollCursorCenterTopBottom,