`this_instr` should be const in generated code. `this_instr` is generated, and should be `const`.

# Bug report

### Bug description:

We currently generate
```C
_Py_CODEUNIT *this_instr =...
```
we should generate 
```C
_Py_CODEUNIT* const this_instr =...
```
to prevent it being accidentally changed.

### CPython versions tested on:

CPython main branch

### Operating systems tested on:

_No response_


### Linked PRs
* gh-125071