Use posix paths with patterns passed to globby.

globby uses the fast-glob package, which states this in its README (RE:
patterns):

> ⚠️ Always use forward-slashes in glob expressions (patterns and ignore
> option). Use backslashes for escaping characters.

Previously, we were using `path.join` which will use the OS specific
separator and therefore fail to match files when invoked with a
directory path directly.

src/runner.js

@@ -1,7 +1,8 @@
 const http = require('http');
 const https = require('https');
 const { writeFileSync } = require('fs');
-const { resolve, extname, join } = require('path');
+const { join } = require('path').posix;
+const { resolve, extname } = require('path');
 const colors = require('colors/safe');
 const globby = require('globby');
 const ora = require('ora');