feat: Add support for ACT_CONFIG_HOME env var

[MatejKafka]

Implement support for a custom config directory through a new ACT_CONFIG_HOME env var. If set, it overrides the default config file locations from XDG and the home dir, but still uses .actrc from the current directory.

With this change, it should be possible to bundle act as a fully portable program by setting ACT_CONFIG_HOME and XDG_CACHE_HOME to a local directory.

[Ziip-dev]

This would be super cool in combination with devbox / nix env to enable more isolated development environments!