install-ffmpeg-common.ps1: Wait only for Bash process to finish

When building FFmpeg for MSVC, the toolchain may boot up other
processes for telemetry. These processes are long-running and outlive
the actual compilation. When we run MSYS with -Wait, we end up stalling
until all sub-processes shut down. This was observed when running this
script locally for MSVC, and caused compilation to freeze.

This patch makes us only wait for the Bash process spawned by MSYS to
finish.

Pick-to: 6.12
Change-Id: Iff89db8e7d3785713e1124b6169cd8a1230eefbc
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>