fix: panic on completions (#1010)

* fix(#1006): panic on completions

as far as i can tell, this was broken by the clap upgrade (#991).

* fix: add smoke test for completions

* skip test on windows

* Create dull-bottles-try.md

---------

Co-authored-by: Gal Schlezinger <gal@spitfire.co.il>

.changeset/dull-bottles-try.md

@@ -0,0 +1,5 @@
+---
+"fnm": patch
+---
+
+fix: panic on `fnm completions`

src/commands/completions.rs

@@ -24,7 +24,8 @@ impl Command for Completions {
             .or_else(|| infer_shell().map(Into::into))
             .ok_or(Error::CantInferShell)?;
         let shell: ClapShell = shell.into();
-        let app = Cli::command();
+        let mut app = Cli::command();
+        app.build();
         shell.generate(&app, &mut stdio);
         Ok(())
     }
@@ -49,3 +50,18 @@ fn shells_as_string() -> String {
         .collect::<Vec<_>>()
         .join("\n")
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    #[cfg(not(windows))]
+    fn test_smoke() {
+        let config = FnmConfig::default();
+        Completions {
+            shell: Some(Shells::Bash),
+        }
+        .call(config);
+    }
+}