revup -f fails if no env.bat/ps1 exists

[paroxysm]

The "append_env" function tries to write to these files but does not pass the create flag in order to ensure they get created if they're missing. W/o the create flag, if those files are missing, you get a cryptic
Critical error, aborting The system cannot find the file specified. (os error 2)

To remedy this, simply modify the file open options to add the create flag like so:
let mut env_file = std::fs::OpenOptions::new().create(true).append(true).open("env.bat")?;