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/wrapper/run.go

@@ -70,9 +70,15 @@ func Run(tool string, args []string) int {
 	var exitCode int
 	switch {
 	case s.rawStdout:
-		// MSBuild: skip all filtering/toolrelay, inherit stdio directly.
+		// MSBuild: skip all filtering/toolrelay, inherit stdio directly, and
+		// add the extra environment MSBuild's own toolset/SDK-detection
+		// props need on top of the generic INCLUDE/LIB/WINEPATH.
 		cmd := exec.Command(wineBin, append([]string{toolExePath}, rewritten...)...)
-		cmd.Env = buildEnv(paths)
+		env := buildEnv(paths)
+		for k, v := range msbuildEnv(cfg, paths) {
+			env = append(env, k+"="+v)
+		}
+		cmd.Env = env
 		cmd.Stdin = os.Stdin
 		cmd.Stdout = os.Stdout
 		cmd.Stderr = os.Stderr