Fix crash if $PWD is used as for-loop variable

for PWD in foo; true; end

prints:

>..src/parse_execution.cpp:461: end_execution_reason_t parse_execution_context_t::run_for_statement(const ast::for_header_t&, const ast::job_list_t&): Assertion `retval == ENV_OK' failed.

because this used the wrong way to see if something is read-only.

tests/checks/basic.fish

@@ -500,3 +500,11 @@ echo banana
 # This used to be a parse error - #7685.
 echo (echo hello\\)
 # CHECK: hello\
+
+# Should fail because $PWD is read-only.
+for PWD in foo bar
+    true
+end
+# CHECKERR: {{.*}}/basic.fish (line {{\d+}}): You cannot use read-only variable 'PWD' in a for loop
+# CHECKERR: for PWD in foo bar
+# CHECKERR: ^