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).

[set-build-dir source code](https://cs.opensource.google/fuchsia/fuchsia/+/main:tools/devshell/set-build-dir)