Set TMPDIR, TMP and TEMP env vars early during launch before domains are loaded

Fix for https://github.com/adobe/brackets/issues/5513.

I found an alternate fix from #358 by setting setting the env vars in node instead of in the native shell code. `osenv` memo-izes `tmpdir` before I can patch the environment vars in `ExtensionManagerDomain.js` in core code, so that's why I chose to do this earlier in `Launcher.js` in the shell.

@ingorichter or @JeffryBooher what do you think of this proposal vs. #358? Not as easy as I would have liked. I was hoping to avoid shell changes.