Unify --sia-directory flag. Add SIA_DATA_DIR env variable.

MERGE REQUEST

Overview

This MR adds a new SIA_DATA_DIR environment variable that allows a user to set the data directory used for Sia.

It also unifies the usage of the --sia-directory flag across siac and siad. The command-line flag overrides the environment variable.

This unification means that apipassword is stored along with the rest of the siad data. By default it's stored in build.DefaultSiaDir() (on Linux ~/.sia).

Tested manually, by setting the data directory using just the flag, just the environment variable, both, and neither from an unrelalated directory.

Example for Visual changes (ie Screenshot)

Issues Closed

Checklist

Review and complete the checklist to ensure that the MR is complete before assigned to an approver.

• All new methods, or updating methods have clear docstrings
• Testing added or updated for new methods
• Any new packages are added to Makefile and .gitlab-ci.yml
• API documentation updated for API updates
• Module README.md updated for changes to work flow
• Issue added to Sia-UI repo for new supporting features
• Changelog updated