Add an option to disable the update check

[tedmielczarek-fastly]

Every time I look at a CI log I see the A new version of the Fastly CLI is available message, because we only update the version we use as-needed.

I would like it if the CLI had an option of some sort to disable this check. An environment variable or config option would be perfect. A commandline option would work, but would mean I would have to remember to include it in every CLI invocation in CI.

From looking at the code I noticed that passing --quiet does silence this output, but in CI I explicitly use --verbose so that the logs contain enough information to diagnose issues that crop up.

[kpfleming]

We already support a ttl attribute in config.toml for wasm-tools and Viceroy, and if that is set properly then no update checks are performed. Supporting this in the cli block in that file should be straightforward.

(edited: this would go into the cli block, not the fastly block)