termio/exec: call wait on subprocess when it exits externally

src/termio/Exec.zig

@@ -1199,6 +1199,10 @@ const Subprocess = struct {
     /// Called to notify that we exited externally so we can unset our
     /// running state.
     pub fn externalExit(self: *Subprocess) void {
+        if (self.command) |*cmd| {
+            _ = cmd.wait(false) catch |err|
+                log.err("error waiting on subprocess: {}", .{err});
+        }
         self.command = null;
     }