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-build-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).