fixed global Mono MSBuild version reporting

[filipw]

This commit 2bfebbc contains a bug where the global Mono MSBuild version was always reported the same as the bundled MSBuild.

Running on global Mono 6.12, before the changes:

[info]: OmniSharp.Stdio.Host
        Starting OmniSharp on MacOS 10.15.6 (x64)
[info]: OmniSharp.Services.DotNetCliService
        DotNetPath set to dotnet
[info]: OmniSharp.MSBuild.Discovery.MSBuildLocator
        Located 2 MSBuild instance(s)
            1: Mono 16.8.0 - "/Library/Frameworks/Mono.framework/Versions/6.12.0/lib/mono/msbuild/Current/bin"
            2: StandAlone 16.8.0 - "/Users/filipw/.vscode/extensions/ms-dotnettools.csharp-1.23.4/.omnisharp/1.37.3/omnisharp/.msbuild/Current/Bin"

After the change:

[info]: OmniSharp.Stdio.Host
        Starting OmniSharp on MacOS 10.15.6 (x64)
[info]: OmniSharp.Services.DotNetCliService
        DotNetPath set to dotnet
[info]: OmniSharp.MSBuild.Discovery.MSBuildLocator
        Located 2 MSBuild instance(s)
            1: Mono 16.6.0 - "/Library/Frameworks/Mono.framework/Versions/6.12.0/lib/mono/msbuild/Current/bin"
            2: StandAlone 16.8.0 - "/Users/filipw/.vscode/extensions/ms-dotnettools.csharp-1.23.4/.omnisharp/1.37.3/omnisharp/.msbuild/Current/Bin"