Allow location of .NET 4.8 installer to be specified

### Checklist

- [X] I have verified this is the correct repository for opening this issue.
- [X] I have verified no other issues exist related to my request.

### Is Your Feature Request Related To A Problem? Please describe.

On older OSes, .NET Framework 4.8 is _not_ the default installed framework. With Chocolatey 2._x_+, we have changed to 4.8 being the targeted framework. This requires a fresh installation, or an upgrade, to reach out to the internet to download the 4.8 installer and execute it.

Reboot requirements not withstanding, this is problematic for regulated environments where access to internet-facing resources is blocked or severely limited.

### Describe The Solution. Why is it needed?

I propose that we introduce a new environment variable `$env:DotnetDownloadUrl` or similar which someone looking to install can use to point to a copy of the installer they have retrieved and made available on their network.

This would unblock them from installing Chocolatey v2.x+

### Additional Context

_No response_

### Related Tickets and Issues

* [Zendesk ticket](https://chocolatey.zendesk.com/agent/tickets/21324)

No file, test, or entry point is named in the issue. Start by tracing the Chocolatey installation path that downloads the .NET Framework 4.8 installer, then inspect related tests; done means an installer location can be supplied for restricted networks while the existing default behavior remains available.

Written by the indexing model from the issue text.