[winsdk] Fix a bug in winsdk script's JSON output

This path fixes an issue that the script append the path separator in
the wrong location.

Bug: 99600
Change-Id: I785a8017552724d06c628a7fc7d798150ff8e215
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/708488
Fuchsia-Auto-Submit: Haowei Wu <haowei@google.com>
Reviewed-by: Petr Hosek <phosek@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Commit-Queue: Haowei Wu <haowei@google.com>

Author: zeroomega

tools/winsdk/winsdk.go

@@ -474,7 +474,9 @@ func addEnvSetup(files *[]packedFile, vcToolsPath string) (string, error) {
 		{"Windows Kits", WinKitVersion, "UnionMetadata", sdkVersion},
 	}
 	// Common entries to all platforms.
-	vcToolsInstallDir := append(vcToolsParts, string(filepath.Separator))
+	// vcToolsInstallDir needs to end with a path separator.
+	vcToolsInstallDir := vcToolsParts
+	vcToolsInstallDir[len(vcToolsInstallDir)-1] += string(filepath.Separator)
 	env := map[string][][]string{
 		"VSINSTALLDIR":      {{`.\`}},
 		"VCINSTALLDIR":      {{`VC\`}},