For cases where users want to be extra careful about not overwriting
manual changes, add a flag to hard-fail. This is only useful if the etag
cache is persistent or otherwise reliable. This flag should not be used
in ephemeral CI workers that won't persist the cache.
Updates https://github.com/tailscale/corp/issues/22177
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>
ad038f4046