scripts: update_chroot: fix bool default logic

BUG=b:299486687
TEST=`update_chroot --skip-toolchain-upgrade` skips cros_setup_toolchains
TEST=`update_chroot` runs cros_setup_toolchains

Change-Id: Iee7cf265c44b65663d59e6b60f7e929e81b914e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4850927
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Jordan Abrahams-Whitehead <ajordanr@google.com>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Jordan Abrahams-Whitehead <ajordanr@google.com>
diff --git a/scripts/update_chroot.py b/scripts/update_chroot.py
index 9f83f35..bb27667 100644
--- a/scripts/update_chroot.py
+++ b/scripts/update_chroot.py
@@ -63,15 +63,15 @@
     group.add_argument(
         "--skip-toolchain-update",
         dest="update_toolchain",
-        action="store_true",
-        default=False,
+        action="store_false",
+        default=True,
         help="Don't update toolchain automatically.",
     )
     group.add_argument(
         "--skip_toolchain_update",
         dest="update_toolchain",
-        action="store_true",
-        default=False,
+        action="store_false",
+        default=True,
         deprecated=deprecated % "--skip-toolchain-update",
         help=argparse.SUPPRESS,
     )