Don't fire variable set event before entering a for-loop

Since #4376, for-loops would set the loop variable outside, so it
stays valid.

They did this by doing the equivalent of

```fish
set -l foo $foo
for foo in 1 2 3
```

And that first imaginary `set -l` would also fire a set-event.

Since there's no use for it and the variable isn't actually set, we
remove it.

Fixes #8384.

src/parse_execution.cpp

@@ -458,18 +458,17 @@ end_execution_reason_t parse_execution_context_t::run_for_statement(
                             for_var_name.c_str());
     }
 
-    // We fire the same event over and over again, just construct it once.
-    event_t evt = event_t::variable_set(for_var_name);
     auto &vars = parser->vars();
     int retval;
     retval = vars.set(for_var_name, ENV_LOCAL | ENV_USER, var ? var->as_list() : wcstring_list_t{});
     assert(retval == ENV_OK);
-    // TODO: For historical reasons we fire here as well, I'm not sure that makes sense?
-    event_fire(*parser, evt);
 
     trace_if_enabled(*parser, L"for", arguments);
     block_t *fb = parser->push_block(block_t::for_block());
 
+    // We fire the same event over and over again, just construct it once.
+    event_t evt = event_t::variable_set(for_var_name);
+
     // Now drive the for loop.
     for (const wcstring &val : arguments) {
         if (auto reason = check_end_execution()) {

tests/checks/for.fish

@@ -32,3 +32,14 @@ begin
 end
 echo $k
 # CHECK: global
+
+function foo --on-variable foo
+    echo foo set
+end
+
+for foo in 1 2 3
+    true
+end
+# CHECK: foo set
+# CHECK: foo set
+# CHECK: foo set