fix(builder): Fully recurse when building

Besides addressing the panic from assuming things were built when they
weren't, this should fix some completion issues for some people.

Fixes #3669

src/build/command.rs

@@ -4008,11 +4008,15 @@ impl<'help> App<'help> {
     /// Call this on the top-level [`Command`] when done building and before reading state for
     /// cases like completions, custom help output, etc.
     pub fn build(&mut self) {
+        self._build_recursive();
+        self._build_bin_names_internal();
+    }
+
+    pub(crate) fn _build_recursive(&mut self) {
         self._build_self();
         for subcmd in self.get_subcommands_mut() {
-            subcmd._build_self();
+            subcmd._build_recursive();
         }
-        self._build_bin_names_internal();
     }
 
     pub(crate) fn _build_self(&mut self) {

tests/builder/tests.rs

@@ -408,3 +408,19 @@ fn mut_arg_all() {
         cmd = cmd.mut_arg(arg_name, |arg| arg.hide_possible_values(true));
     }
 }
+
+#[test]
+fn issue_3669_command_build_recurses() {
+    let mut cmd = Command::new("ctest").subcommand(
+        Command::new("subcmd").subcommand(
+            Command::new("multi")
+                .about("tests subcommands")
+                .author("Kevin K. <kbknapp@gmail.com>")
+                .version("0.1")
+                .arg(clap::arg!(
+                    <FLAG>                    "tests flags"
+                )),
+        ),
+    );
+    cmd.build();
+}