Add Support for setting CLI Flags

[abegehr]

It would be great to have an option to set cli flags on the astro build command.
Specifically, I'd like to specify the --config flag to deploy a static version of my site.
Reference: https://docs.astro.build/en/reference/cli-reference/

[natemoo-re]

I like the idea but it might be better solved by an environment variable? That would require an upstream change to astro to support something like ASTRO_CONFIG_PATH as well as --config. That would be more secure than allowing an arbitrary string to be executed (which could potentially allow an attacker to exfiltrate private tokens via PR.)