[third_party/ffmpeg] update paths in roll scripts

This CL updates paths used in ffmpeg roll scripts. Previously, the
roll scripts assumed this structure:

    third_party/ffmpeg      fuchsia ffmpeg repo
    third_party/ffmpeg/src  chromium ffmpeg repo

We are actually using this structure:

    build/secondary/third_party/ffmpeg    fuchsia ffmpeg repo
    third_party/ffmpeg/src                chromium ffmpeg repo

The scripts have been updated accordingly.

Fixed: 114726

Test: build change, manually tested
Change-Id: If6cb330f8ea072aca58e32a824d098b566f7b67e
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/ffmpeg/+/767545
Reviewed-by: Petr Hosek <phosek@google.com>
8 files changed