nixpkgs has hardcoded references to Nix versions, we need to provide
it with something that looks like a Nix version while actually being a
Tvix version.
For now, we do this by stealing a trick out of the browser book and
constructing a version that looks like a Nix version to Nix, but like
a Tvix version to people who know what they are looking for.
Nevermind that we don't actually have any kind of versioning for
Tvix (yet?), other than depot revisions.
Change-Id: I7ce8079dd8164a2079891d38e707f09a45f0bbc1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6858
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
