Refactor usage of system commands to a module w/ autodiscovery or manually set $PREFIX?

[ermo]

Currently, there are a few direct command invocations in solbuild, including calls to /bin/su.

To make it easier to update solbuild for the upcoming /usr merge task, consider how to consolidate known commands to a single module that adds (or discovers at startup?) the preferred location of commands.

The motive for possibly considering dynamic discovery of commands is that hard-coding commands is not really desirable if we want for solbuild to keep working outside of solus.