Don't create msi links for windows gnullvm targets

[arnabanimesh]

Since msi installers for windows gnullvm targets are not being published, it should not be part of the documentation.

Ideally it would be better if there would be msi installers for these targets in my opinion though.

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[apiraino]

@arnabanimesh just to understand the context of this patch. The fact that the .msi Windows packages are not available for that target is intentional from us or is it something we should look into?

A bit more context in the description would be helpful for those (like me) in the project not extremely familiar about our releases artifacts. Thanks.

[arnabanimesh]

@apiraino Added proper explanation in the description

[apiraino]

I was curious so I asked: https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/No.20Windows.20.2Emsi.20packages.20for.20targets.20.60-gnullvm.60/near/553825621

Please let's wait a sec before merging this.

@rustbot label S-blocked

[Urgau]

We have got an answer for why we don't yet produce .msi installers for the gnullvm targets, bc we cross-compile from Linux and cannot create an .msi installers from there.

Until that's done (cc rust-lang/rust#148751), I think we can move along, and when the situation changes we can revert this PR.

@rustbot label -S-blocked

[Urgau]

LGTM, thanks for bringing this up.