fix: output dir is dir

[SiyaoIsHiding]

In this code snippet:

let OutputDir = getRunnerInput('output_dir', 'built', getRunnerPath);
let OutputDirIsDir = false
try {
    OutputDirIsDir = fs.lstatSync(OutputDir).isDirectory();
} catch { }
if (!OutputDirIsDir) {
    OutputDir += OutputDir.endsWith("/") ? "" : "/"
    CreateOutputDirectory(OutputDir);
}

The current logic is - if the output directory is not a directory, then ensure it ends with a slash.
The logic should be - if the output directory is a directory, then ensure it ends with a slash.

[SiyaoIsHiding]

@BaileyJM02 , just in case u didn't receive the notification on the pull request :)