vscode/tasks.json: Be explicit about using 'cmd'

... for the shell.

Without being explicit, the shell used for these tasks is the one specified in the user's config.
If the default shell is non-powershell, then these tasks silently fail to execute.

Change-Id: I7916a3461a3d561d669bd94e1bf32da1b60d0b69
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/58223
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Auto-Submit: Ben Clayton <bclayton@google.com>

.vscode/tasks.json

@@ -40,13 +40,15 @@
             "windows": {
                 // Invokes ninja in the 'out/active' directory, which is created
                 // with the 'generate' task (see below).
-                "command": "cmd",
+                "command": "/C",
                 "args": [
-                    "/C",
                     "ninja && echo Done"
                 ],
                 "options": {
                     "cwd": "${workspaceRoot}/out/active",
+                    "shell": {
+                        "executable": "cmd"
+                    }
                 },
             },
             "presentation": {
@@ -100,11 +102,15 @@
                 // is_debug argument set to to true iff the build-type is Debug.
                 // A symbolic link to this build directory is created at 'out/active'
                 // which is used to track the active build directory.
-                "command": "cmd",
+                "command": "/C",
                 "args": [
-                    "/C",
                     "(IF \"${input:buildType}\" == \"Debug\" ( gn gen 'out\\${input:buildType}' --args=is_debug=true ) ELSE ( gn gen 'out\\${input:buildType}' --args=is_debug=false )) && (IF EXIST 'out\\active' rmdir 'out\\active' /q /s) && (mklink /j 'out\\active' 'out\\${input:buildType}')",
                 ],
+                "options": {
+                    "shell": {
+                        "executable": "cmd"
+                    }
+                },
             },
             "problemMatcher": [],
         },