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

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;
  }
  }