Nixos policy is not to ping home by default, so make the update check
default to false.
It can still be re-enabled by config or env var if required:
- `check-for-app-update: true` in a `.syft.yaml`
- SYFT_CHECK_FOR_APP_UPDATE=true
This can be verified by checking for network connections when
trying to scan a non-existing file (or `toolbox-data.anchore.io` dns
requests):
`strace -f -e connect syft scan a 2>&1 | grep AF_INET`