WindowsSdk: Fix finding the _Instances directory when it's not in the default location

[squeek502]

Information about installed MSVC instances are stored in state.json files within a Packages/_Instances directory. The default location for this is %PROGRAMDATA%\Microsoft\VisualStudio\Packages\_Instances. However, it is possible for the Packages directory to be put somewhere else. In that case, the registry value HKLM\SOFTWARE\Microsoft\VisualStudio\Setup\CachePath is set and contains the path to the Packages directory.

Previously, WindowsSdk did not check that registry value. After this commit, the registry value HKLM\SOFTWARE\Microsoft\VisualStudio\Setup\CachePath is checked first, which matches what ISetupEnumInstances does (according to a Procmon log).

Some more context: https://ziggit.dev/t/how-to-correctly-compile-static-libraries-on-windows/4232/23

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Also fixes KEY_WOW64_32KEY being set whenever opening a registry key. This was causing HKLM\SOFTWARE\WOW6432Node\Microsoft\VisualStudio\Setup to be attempted to be opened instead of HKLM\SOFTWARE\Microsoft\VisualStudio\Setup

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Confirmed to fix the motivating use case: https://ziggit.dev/t/how-to-correctly-compile-static-libraries-on-windows/4232/34

[perillo]

@squeek502

Is the use of break :method<n> in the findInstancesDir function really necessary?
I think that a simple use of scoping should be enough.

Original

method1: {
    return findInstancesDirViaSetup(allocator) catch |err| switch (err) {
        error.OutOfMemory => |e| return e,
        error.PathNotFound => break :method1,
    };
}

Alternate (not tested)

{
    return findInstancesDirViaSetup(allocator) catch |err| switch (err) {
        error.OutOfMemory => |e| return e,
        error.PathNotFound => {},
    };
}

[squeek502]

The labeled blocks aren't necessary, but I thought they made things a bit more clear.