6.x - Fix vstudio exporter

[ohthesetrees]

What does this PR do?

Fixes the vstudio exporter. When I ran it by default I got an error:

Configuring...
Exporting Premake...
Error: C:/Dev/premake-core/core/modules/tree/tree.lua:100: attempt to index a nil value (local 'self')
stack traceback:
        ...e-core/modules/exporters/vstudio/src/vcxproj.filters.lua:41: in field 'export'
        ...v/premake-core/modules/exporters/vstudio/src/vcxproj.lua:266: in field 'export'
        C:/Dev/premake-core/modules/exporters/vstudio/vstudio.lua:282: in function 'vstudio.exportProject'
        C:/Dev/premake-core/modules/exporters/vstudio/vstudio.lua:271: in function 'vstudio.exportWorkspace'
        C:/Dev/premake-core/modules/exporters/vstudio/vstudio.lua:54: in function 'vstudio.export'
        C:/Dev/premake-core/modules/exporters/vstudio/_register.lua:9: in field 'execute'
        C:/Dev/premake-core/core/modules/options/options.lua:90: in function 'options.execute'
        C:/Dev/premake-core/core/modules/main/main.lua:67: in function 'main.executeCommandLineOptions'
        C:/Dev/premake-core/core/modules/premake/premake.lua:57: in function 'premake.callArray'
        C:/Dev/premake-core/core/modules/main/main.lua:88: in function 'main.run'
        C:/Dev/premake-core/core/host/src/_premake_main.lua:20: in function '_premake_main'

It seems like the virtualSourceTree property should actually have been sourceTree. I'm not 100% confident but after this change I was able to generate a solution and build it.

How does this PR change Premake's behavior?

Fixes the vstudio exporter.

Anything else we should know?

I'm not 100% certain on the changes. All I know is I couldn't generate a project before and after making this change I can so it seems correct, but please let me know if this is wrong for some reason.

Did you check all the boxes?

• [x] Focus on a single fix or feature; remove any unrelated formatting or code changes
• [x] Add unit tests showing fix or feature works; all tests pass
• [x] Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• [x] Follow our coding conventions
• [x] Minimize the number of commits
• [x] Align documentation to your changes

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[nickclark2016]

@kellygravelyn Without the changes, can you try setting the PREMAKE_PATH env var to your v5 source directory when trying to generate VS files? This is a discussion I've had with @starkos wayyyy in the past when I was first trying to get stood up on v6. If this is a more permanent solution, that'd be awesome.