libvpx: 1.6.1 -> 1.7.0

Semi-automatic update generated by https://github.com/ryantm/nix-update tools. These checks were done:

- built on NixOS
/tmp/tmp.Z8QazLD8jI /home/ryantm/.cache/nixpkgs
/home/ryantm/.cache/nixpkgs
- ran `/nix/store/q26np2w8fjj2cjsgkhklbywd4vvykq67-libvpx-1.7.0-bin/bin/vpxdec --help` got 0 exit code
- ran `/nix/store/q26np2w8fjj2cjsgkhklbywd4vvykq67-libvpx-1.7.0-bin/bin/vpxdec --help` and found version 1.7.0
- ran `/nix/store/q26np2w8fjj2cjsgkhklbywd4vvykq67-libvpx-1.7.0-bin/bin/vpxenc --help` got 0 exit code
- ran `/nix/store/q26np2w8fjj2cjsgkhklbywd4vvykq67-libvpx-1.7.0-bin/bin/vpxenc --help` and found version 1.7.0
- found 1.7.0 with grep in /nix/store/q26np2w8fjj2cjsgkhklbywd4vvykq67-libvpx-1.7.0-bin
- directory tree listing: https://gist.github.com/feae181f96e3cbf1235f1f3b9503500c