Build fixes for CGO

[darthShadow]

This is trying to fix a few build errors due to cgo constraints and incorrect workflow for testing that.

[darthShadow]

You can see the current workflow run here: https://github.com/darthShadow/go-ieproxy/actions/runs/3066692597

It's currently failing tests for darwin & windows when CGO is disabled:

• The darwin test is failing because of the missing implementation for overrideEnvWithStaticProxy in ieproxy_unix.go
• The windows test is failing because of the missing regedit defintions in ieproxy_unix.go

[darthShadow]

Hi, Is there any problem with this PR that I need to fix that would allow it to get merged?

[djs55]

I think the Windows code doesn't use cgo (no import "C") so it's ok to build it without cgo: djs55@39db71e

[darthShadow]

Ah, I missed that bit. Do you want me to cherry-pick your commit or do you want to create a separate PR once/if this is merged?

[s-weigand]

Any update on this? The windows support of an upstream project seems to depend on it 🥺

[mattn]

Sorry, could you please rebase master branch?