Add flag to disable Syft hitting toolbox-data.anchore.io

[cpendery]

What would you like to be added:
A flag to disable Syft's checking of the most recent version of Syft to prompt for potential update

Why is this needed:
Syft tries to hit Anchore's s3 toolbox on every run, which causes a long delay when running in an air gapped environment.

[0030] ERROR failed to fetch latest version: Get "https://toolbox-data.anchore.io/syft/releases/latest/VERSION": dial tcp 111.11.1.111:443: i/o timeout

Additional Information
Grype provides this under the GRYPE_DB_AUTO_UPDATE environment variable

[joshbressers]

I think this exists

# enable/disable checking for application updates on startup
# same as SYFT_CHECK_FOR_APP_UPDATE env var
check-for-app-update: true

https://github.com/anchore/syft/blob/main/README.md?plain=1#L427