bug 2020056: Fix Glean log pipe corruption on Windows

[ahochheiden]

D283734 moved telemetry initialization to a separate thread on mach startup and uncovered this. On Windows, sometimes the logging shows up character by character on the MSYS2 terminal like so:

      0 [waitproc] bash 2253 proc_waiter: unknown value 123 on proc pipe
    114 [waitproc] bash 2253 proc_waiter: unknown value 34 on proc pipe
    199 [waitproc] bash 2253 proc_waiter: unknown value 108 on proc pipe
    256 [waitproc] bash 2253 proc_waiter: unknown value 101 on proc pipe
<sic>

It's not 100% reproducible, since the handle value the file descriptor mapped to isn't always one that produces visible output, but after applying this fix the symptom has yet to reappear for me locally.