Reverts "Bump channel to a NixOS 20.03 release commit".
This reverts commit a629d7fdd4.
This commit moves to a 20.03 release commit, in which building
Chromium with VAAPI is broken.
Externally developed dependencies which have been imported ("vendored") into
this repository. These dependencies come with their own licenses and whatever
else.
Code that is developed inside of this repository, but released to an external
repository via Copybara.