🐛 redownload-electron-bins.js: Correct path

Construct the atomRepoPath variable starting with __dirname,
so that atomRepoPath is an absolute path.

For example: '/Users/[username]/atom/package.json',
versus '../../../../atom/package.json'.

---

Explanation for why this is necessary:

The require() and fs.existsSync() functions interpret
relative paths differently.

require() is relative to the current script's containing folder,
(which is the same as __dirname),
whereas 'fs.existsSync()' is relative to the working directory
(which is the same as process.cwd()).

When constructing the path to the Atom repo's main package.json,
we should construct it as an absolute path, for consistency's sake.
Otherwise the script will error out, or use the wrong package.json.

script/redownload-electron-bins.js

@@ -12,7 +12,7 @@ if (typeof(downloadMksnapshotPath) !== 'undefined') {
   const path = require('path');
   const fs = require('fs');
 
-  const atomRepoPath = path.join('..', '..', '..', '..', 'atom', 'package.json');
+  const atomRepoPath = path.join(__dirname, '..', '..', '..', '..', 'atom', 'package.json');
   const electronVersion = fs.existsSync(atomRepoPath) ? require(atomRepoPath).electronVersion : '6.1.12'
   // TODO: Keep the above "electronVersion" in sync with "electronVersion" from Atom's package.json