argparse: Fix validation for short-only-flags

This read something like `o=!_validate_int`, and the flag modifier
reading kept the pointer after the `!`, so it created a long flag
called `_validate_int`, which meant it would not only error out form

```fish
argparse 'i=!_validate_int' 'o=!_validate_int' -- $argv
```

with "Long flag '_validate_int' already defined", but also set
$_flag_validate_int.

Fixes #5864.

src/builtin_argparse.cpp

@@ -181,6 +181,8 @@ static bool parse_flag_modifiers(const argparse_cmd_opts_t &opts, const option_s
     if (*s == L'!') {
         s++;
         opt_spec->validation_command = wcstring(s);
+        // Move cursor to the end so we don't expect a long flag.
+        while (*s) s++;
     } else if (*s) {
         streams.err.append_format(BUILTIN_ERR_INVALID_OPT_SPEC, opts.name.c_str(),
                                   option_spec.c_str(), *s);