Use --frontend_file for ninja instead of cat

Ninja now knows how to write directly to a file (or in our case, a named
pipe). This works around an issue we were seeing on Mac, where Go would
just hang after 50-2000 proto messages. It's also just a simpler
solution.

Bug: 111544015
Test: `m` with updated ninja on both Linux & Mac
Change-Id: Ic91920d83a6d2ea0b79e82b467e2423d78189f12