set the default build directory used by other fx commands
usage: fx set-build-dir DIR or: fx --dir DIR set-build-dir or: fx set-buil-dir 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 set-build-dir` later. fx set-build-dir without arguments will list the available build directories, naming the current active build directory (if any).