Rename GetBuildVersion target to GetVsixVersion

The [Nerdbank.GitVersioning](https://github.com/AArnott/Nerdbank.GitVersioning) package defines a `GetBuildVersion` target: https://github.com/AArnott/Nerdbank.GitVersioning/blob/d398833fcdcb857fe44c3e8287f25b4058921181/src/Nerdbank.GitVersioning.NuGet/build/Nerdbank.GitVersioning.targets#L62

This conflicts with the `GetBuildVersion` target which repo toolset defines if `UsingToolVSSDK` is set to true. The result is that if you try to enable the VS SDK in a project, then Nerdbank.GitVersioning will stop working.

PR #132 allows this to be worked around by setting the `DefineGetBuildVersionTargetForVsix` property to false. However, a better fix would be to rename `GetBuildVersion` to `GetVsixVersion` in repo toolset.

This would be a breaking change for consumers of repo toolset- they'd have to update their vsix definitions to use the new target name.