revert: "feat(dogfood): install nix package manager"

[mafredri]

Reverts #5308.

After merging #5308, I noticed bash /opt/nix/install --no-daemon never completes. It gets stuck here:

performing a single-user installation of Nix...
copying Nix to /nix/store............................................
replacing old 'nix-2.3.15'
installing 'nix-2.3.15'
unpacking channels...

I've tried waiting for a good while. I tried upgrading nix to 2.13.2. I tried running the install as sudo. Same result every time.

Since I don't know the fix or cause, I'm reverting (to avoid breakage) until we can figure it out.