Make pw_target_toolchain a build arg on Windows

This makes Windows consistent with the other host platforms and this
behavior is required by CI builds.

Change-Id: I35b3df2d82364d3d43f6c334e71e7bef8d8a0f4d
diff --git a/targets/host/windows.gni b/targets/host/windows.gni
index a35c9f4..b287665 100644
--- a/targets/host/windows.gni
+++ b/targets/host/windows.gni
@@ -16,7 +16,9 @@
 
 pw_automatic_test_runner = get_path_info("run_test.bat", "abspath")
 
-pw_target_toolchain = "$dir_pw_toolchain:host_gcc_og"
+declare_args() {
+  pw_target_toolchain = "$dir_pw_toolchain:host_gcc_og"
+}
 
 # This is here as bloaty_config_file cannot be an empty string or GN fails.
 # TODO(frolv): Add this file and enable size reports on Windows.