fix(complete): Fallback to rustc if rustup fails for --target completions

If there is any problem with rustup, we should fallback to rustc.

(this also removes some extra allocations)

Author: epage

src/cargo/util/command_prelude.rs

@@ -1101,11 +1101,13 @@ fn get_target_triples() -> Vec<clap_complete::CompletionCandidate> {
 
     if is_rustup() {
         if let Ok(targets) = get_target_triples_from_rustup() {
-            candidates.extend(targets);
+            candidates = targets;
         }
-    } else {
+    }
+
+    if candidates.is_empty() {
         if let Ok(targets) = get_target_triples_from_rustc() {
-            candidates.extend(targets);
+            candidates = targets;
         }
     }