Also set the read-only flag for non-electric vars

For some reason, we have two places where a variable can be read-only:

- By key in env.cpp:is_read_only(), which is checked via set*

- By flag on the actual env_var_t, which is checked e.g. in
  parse_execution

The latter didn't happen for non-electric variables like hostname,
because they used the default constructor, because they were
constructed via operator[] (or some such C++-iness).

This caused for-loops to crash on an assert if they used a
non-electric read-only var like $hostname or $SHLVL.

Instead, we explicitly set the flag.

We might want to remove one of the two read-only checks, or something?

Fixes #5548.

src/env.cpp

@@ -1221,6 +1221,7 @@ int env_stack_t::set_internal(const wcstring &key, env_mode_flags_t input_var_mo
 
             var.set_vals(std::move(val));
             var.set_pathvar(var_mode & ENV_PATHVAR);
+            var.set_read_only(is_read_only(key));
 
             if (var_mode & ENV_EXPORT) {
                 // The new variable is exported.