testplan: Replace `os.WriteFile` with `ioutil.WriteFile`.

- `os.WriteFile` was introduced in go 1.16, chroot has a newer
version.

BUG=b:207715948
BUG=b:182898188
TEST=`./fast_build.sh -T`
TEST=`emerge test-plan`

Change-Id: Id90429df7c9bfae9862af43a49580784af30ef3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/3305990
Commit-Queue: Andrew Lamb <andrewlamb@chromium.org>
Commit-Queue: Tim Bain <tbain@google.com>
Commit-Queue: Seewai Fu <seewaifu@google.com>
Tested-by: Andrew Lamb <andrewlamb@chromium.org>
Auto-Submit: Andrew Lamb <andrewlamb@chromium.org>
Reviewed-by: Tim Bain <tbain@google.com>
Reviewed-by: Seewai Fu <seewaifu@google.com>
2 files changed