Render script runs in first open document regardless of window focus
**Expected behavior:** running `npm run render` should run the script in the currently open/focused document.

**Observed behavior:** running `npm run render` runs the script in the first document opened in the current window/session.

**How to reproduce:**
1. Open or create two Sketch documents
2. Focus the second document window/tab
3. Run `npm run render`
4. The second document is unchanged; the first, unfocused document is where the contents are rendered.

**Sketch version:** 43.2