Set root directory to temp directory if not provided

[RebeccaMahany]

Fixes issue where if root dir is not provided, we correctly make a temporary directory to use as the root directory, but do not save the value and promptly try to make a directory with path "":

{"caller":"launcher.go:97","msg":"using default system root directory","path":"/var/folders/0d/l0cmv9pd4qd3j0d98q1st9bm0000gn/T/launcher-root3415220570","severity":"info","ts":"2023-05-09T19:09:56.293064Z"}
{"caller":"main.go:112","creating root directory: mkdir : no such file or directory":"run launcher","severity":"debug","stack":"creating root directory: mkdir : no such file or directory","ts":"2023-05-09T19:09:56.293091Z"}
{"caller":"logutil.go:13","creating root directory: mkdir : no such file or directory":"run launcher","severity":"info","ts":"2023-05-09T19:09:56.293138Z"}

[directionless]

I wonder how long this bug has been there.