Skip constructing SATySFi Root

[na4zagin3]

I'd like to allow users to describe build dependencies with arbitrary build tools (e.g., make, omake, & ninja).

Here is my idea. We'll have a new subcommand build which runs commands with special environment variables SATYROGRAPHOS_PROJECT.

In this case, SATYROGRAPHOS_PROJECT will contain (satysfi_root_dir /path/to/satysfi/root).

When Satyrographos is executed with environment variable SATYROGRAPHOS_PROJECT, it runs in child mode, where satysfi subcommand uses the SATySFi Root Directory specified in the environment variable without constructing a new one.