[llvm] [gn] Use exec_script_allowlist in //llvm/utils/gn/.gn (PR #134172)

[Nico Weber via llvm-commits]

nico wrote:

Hm, looks like the assert runs too late. On a machine where I hadn't bumped GN yet, I'm getting

```
ERROR at //.gn:9:25: Assignment had no effect.
exec_script_allowlist = []
                        ^
You set the variable "exec_script_allowlist" here and it was unused before it went
out of scope.
```

Ah well. If anyone runs into this, running `llvm/utils/gn/get.py` makes it go away.

https://github.com/llvm/llvm-project/pull/134172