Start an external command, defined in args.
The process environment vars are inherited from the parent by
default (use :clear-env to clear them).
If needed, provide options in map as first arg:
:in - a ProcessBuilder.Redirect (default = :pipe) or :inherit
:out - a ProcessBuilder.Redirect (default = :pipe) or :inherit :discard
:err - a ProcessBuilder.Redirect (default = :pipe) or :inherit :discard :stdout
:dir - current directory when the process runs (default=".")
:clear-env - if true, remove all inherited parent env vars
:env - {env-var value} of environment variables to set (all strings)
Returns the java.lang.Process.