Source builds should not try to download releases

[kjeremy]

This would most likely fix #3105 but I think it deserves it's own issue.

[Veetaha]

Hmm, correct me if I am wrong, when you do cargo xtask install vscode still downloads and installs marketplace frontend extension and you do see Download now info message (note that you won't see it if you already installed prebuilt binaries and they are available somewhere in your AppData\Code\User\globalStorage\matklad.rust-analyzer-<something>)?

[kjeremy]

I'm not sure that I follow. Like I said in the other issue if I cargo xtask install it installs everything correctly.

If I pull my ethernet cable and then run vscode the version of the extension is 0.1.0. Once I plug the cable back in and restart vscode (or reload the workspace) the extension becomes: 0.1.20200211 which suggests that it downloads.

[Veetaha]

@kjeremy, did #3136 fix this?