re-use a previous build directory set up by fx set
usage: fx use DIR
or: fx --dir DIR use
or: fx use
Switches further `fx` commands to using a different build directory.
This only works if `fx --dir DIR set ...` succeeded previously
(and DIR has not been removed since). The next `fx build` or other
such command will now refer to DIR. The previous build directory is
left in place, so you can switch back again with `fx use` later.
fx use without arguments will list the available build directories, naming
the current active build directory (if any).