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.

tests/test1.err

@@ -38,6 +38,12 @@ fish: You cannot use read-only variable 'status' in a for loop
 for status in a b c
     ^
 
+####################
+# That goes for non-electric ones as well (#5548)
+fish: You cannot use read-only variable 'hostname' in a for loop
+for hostname in a b c
+    ^
+
 ####################
 # For loop control vars available outside the for block