Fix MSVC build.

Use $shell prefix invoking python for cp.py.

Change-Id: Ic4b40337255a91fe6f9729eb58ecb6d47c559001
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/306334
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>

gn/toolchain/BUILD.gn

@@ -201,7 +201,7 @@ toolchain("msvc") {
 
   tool("copy") {
     cp_py = rebase_path("../cp.py")
-    command = "python \"$cp_py\" {{source}} {{output}}"
+    command = "$shell python \"$cp_py\" {{source}} {{output}}"
     description = "copy {{source}} {{output}}"
   }
 }