Should Environment.GetEnvironmentVariables return case-insensitive collection on Windows?

As pointed by @stephentoub in https://github.com/dotnet/runtime/pull/124271#discussion_r2800533246:

```csharp
var d = Environment.GetEnvironmentVariables();
d.Clear();
d.Add("s", 123);
d.Add("S", 456);
foreach (var v in d)
{
Console.WriteLine(v);
}
```

prints

```log
[S, 456]
[s, 123]
```

rather than

```log
[S, 456]
```

I personally don't have a strong opinion on this, the API just returns a snapshot. However, if we decide to change it, we could eliminate some extra copying when creating env var dictionary for process:

https://github.com/dotnet/runtime/blob/e771b591f55da956c59c2bca8250adde76c608aa/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessStartInfo.cs#L99-L101

cc @GrabYourPitchforks