No UserAgent on windows implants

[C-Sto]

Describe the bug
Implants generated don't appear to have a useragent set. Tracked it down to:
https://github.com/BishopFox/sliver/blame/dc93d054a9e68644203673a48c9b669683e43cb4/server/generate/binaries.go#L485

which should be able to be resolved by returning like return pbC2Implant.UserAgent instead.

This causes the wininet driver specifically to not do any network comms, as it rejects the header being added.

To Reproduce
Steps to reproduce the behavior:
on master branch, generate -G -b https://some-url-here/?driver=wininet -l -R --debug

run implant on windows machine, and see errors along the lines of

2024/02/27 00:36:42 sliver.go:151: [session] failed to establish connection: failed to add request headers: HttpAddRequestHeadersW: winapi error #12150

Expected behavior
an implant with a useragent

Desktop (please complete the following information):

• OS: Windows
• Version: All

Additional context
Unsure if that's the 'correct' way of fixing this bug