Correctly determine the 64-bit "Program Files" folder - fixes #1286

[TeaDrivenDev]

The preferred way here would be just using Environment.SpecialFolder.ProgramFiles and Environment.SpecialFolder.ProgramFilesX86 for the two values, but that would require compiling Fake.exe as AnyCPU (with "Prefer 32-bit" off).

In a 32-bit process, both Environment.SpecialFolder.ProgramFiles and the environment variable ProgramFiles return the x86 folder. That means there is no official API to get the 64-bit Program Files folder from a 32-bit process, other than going to the registry, as mentioned in #1286.

[matthid]

I think this is fine considering FAKE 5 is a new major.
Though we need to think about how those APIs should behave on unixes...