Auto merge of #14564 - epage:rustup, r=weihanglo

fix(complete): Harden `--target` completions

### What does this PR try to resolve?

I found `--target` wasn't working with rustup as I expected, so this fixes it.  We generally only reference rustup if we are running under it but I think in UX code like this, with a fallback scheme, this should be reasonable enough.

### How should we test and review this PR?

### Additional information

Author: bors

src/cargo/util/command_prelude.rs

@@ -1121,13 +1121,13 @@ fn get_targets_from_metadata() -> CargoResult<Vec<Target>> {
 fn get_target_triples() -> Vec<clap_complete::CompletionCandidate> {
     let mut candidates = Vec::new();
 
-    if is_rustup() {
-        if let Ok(targets) = get_target_triples_from_rustup() {
-            candidates.extend(targets);
-        }
-    } else {
+    if let Ok(targets) = get_target_triples_from_rustup() {
+        candidates = targets;
+    }
+
+    if candidates.is_empty() {
         if let Ok(targets) = get_target_triples_from_rustc() {
-            candidates.extend(targets);
+            candidates = targets;
         }
     }