VS2017 path to sn.exe is wrong

[yaakov-h]

See #202 and #482.

VS2017 doesn't install NETFX 4.6 Tools inside %ProgramFiles(x86)\Microsoft SDKs\Windows\v10.0A\bin, but it does install:

• NETFX 4.6.1 Tools
• NETFX 4.6.2 Tools
• NETFX 4.7 Tools
• NETFX 4.7.1 Tools

We could assume we should use 4.6.1 instead, or we could try and resolve this path based on the project's $(TargetFramework), then fall back to an assumption.

I'm still not quite sure how VS resolves the path internally.