Fix the OSArchitecture retrieval in powershell install

Question

PowerShell architecture system var is fragile. On some versions the install is broken. There is a simple change proposed by @thedwindlingdba in #20782. This will also fix #19559

```ps1
...
$architecture = [System.Runtime.InteropServices.RuntimeInformation,mscorlib]::OSArchitecture.ToString().ToLower()
...
switch($architecture) {
    "arm64" { ... }
    "x64" { ... }
}
```

These lines should change (or open the repo for external PRs): https://github.com/openai/codex/blob/a71e040df51ec0e1f5523