Fix MSBuild toolset/SDK detection under Wine

msbuild <project>.vcxproj previously failed with MSB8020 ("build tools for
vNNN cannot be found") because MSBuild's own toolset/SDK resolution reads
a different set of environment variables than cl/link/lib do directly
(VCInstallDir_<N>, VCToolsInstallDir_<N>, VsInstallRoot,
WindowsSdkDir_10, WindowsTargetPlatformVersion, DisableRegistryUse, etc) -
none of which the generic INCLUDE/LIB/WINEPATH env covered.

Added msbuildEnv, populated for every MSBuild toolset generation actually
present under MSBuild/Microsoft/VC/v*, and wired into the msbuild wrapper.

Verified end-to-end: msbuild successfully builds ocornut/imgui's
example_win32_directx11.vcxproj (retargeted from its original v141
PlatformToolset to this install's v145) - compiles all 8 sources and
links against d3d11.lib/d3dcompiler.lib/dxgi.lib, producing a valid
PE32+ executable.

internal/wineenv/paths.go

@@ -16,6 +16,14 @@ type Paths struct {
 	SDKBinDir     string // <dest>/kits/10/bin/<sdkver>/<host> - mc/midl/mt/rc live here
 	MSBuildBinDir string // <dest>/MSBuild/Current/Bin/<dotnetHost> - MSBuild.exe lives here
 
+	// Windows-notation ("z:\...") equivalents of the paths above, needed to
+	// populate the MSBuild-specific environment variables its toolset/SDK
+	// detection props read (see msbuildEnv in the wrapper package).
+	BaseWin     string // z:\<dest>
+	MSVCBaseWin string // z:\<dest>\vc
+	MSVCDirWin  string // z:\<dest>\vc\tools\msvc\<ver>
+	SDKBaseWin  string // z:\<dest>\kits\10
+
 	Include          string
 	Lib              string
 	LibPath          string
@@ -89,6 +97,11 @@ func NewPaths(cfg *Config, baseUnix string) *Paths {
 		SDKBinDir:     sdkBinDir,
 		MSBuildBinDir: msbuildBinDir,
 
+		BaseWin:     winBase,
+		MSVCBaseWin: msvcBase,
+		MSVCDirWin:  msvcDirWin,
+		SDKBaseWin:  sdkBase,
+
 		Include:          include,
 		Lib:              lib,
 		LibPath:          lib,