fix(complete): Always check rustup for --target

clap-rs/clap#5733 removed the rustup proxy so that
`CARGO_COMPLETE=bash cargo +nightly` works
(with a side benefit of removing the proxy overhead).

As a downside, cargo no longer knows it is running within rustup, so we
aren't reading `--target` candidates from rustup.

This changes the code to always try rustup.  It is likely a good enough
source, even if the user isn't currently using it.
The candidates should be about the same, just rustup hides some by
default.
Hiding just means it isn't shown by default but if only hidden
candidates match, then we show them.

Author: epage

src/cargo/util/command_prelude.rs

@@ -1099,10 +1099,8 @@ 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 = targets;
-        }
+    if let Ok(targets) = get_target_triples_from_rustup() {
+        candidates = targets;
     }
 
     if candidates.is_empty() {