win32: reduce size of PATH env var + bump ffmpeg to 4.1.3
gst-uninstalled: De-dedup before prepending to an env var Helps us avoid breaching the maximum length limit for env var values on Windows.
gst-uninstalled: Try to use short names for env vars on Windows Try even harder to not hit the maximum length limit for env var values on Windows. Reduces the size by ~1000 characters on my machine.
ffmpeg.wrap: Bump to 4.1.3 Tested on Windows.