Avoid changing the current working directory

[casey]

Just changes its CWD to the same directory as the justfile, so subprocesses inherit the CWD of the just. I dislike that there are implicit dependencies in the code between where the working directory is changed and where it's used. Ideally, it would be nice to determine the working directory up front, save it in the Config object, and pass it to subcommands using Command::current_dir().

Note: Don't save it in the config object, since the config object should be derived solely from the arguments. Also put invocation_directory somewhere else.

Notes to self:

• test that fails if current dir isn't set correctly, backticks, recipes
• resolve -> resolve_justfile, or search, search::justfile for justfile
• test config parsing, all arguments (also test running?)
• config.run() -> config.subcommand.run()