nix-base base snap broken, because top-level bind-mount /nix blacklisted

[Alexander Nikolaev]

When I tried to build a snap package with nixpkgs's makeSnap tool (which almost a self-contained container image, with absolute paths, which starts from /nix, and have a dependency on nix-base snap for setting up empty directories structure). Also it rely on bind-mount $SNAP/nix -> /nix, which was blacklisted by following commit https://github.com/snapcore/snapd/commit/7ae57afc07c5bb5854525921ad112b2413bb2ee5)

I asked on #snappy IRC, and learn, that there was some agreement about whitelisted base snaps, which allow extra top-level bind-mounts, including nix-base.

Citing from IRC:
13:48 < zyga> avn, it's a bug in snapd
13:48 < zyga> mborzecki, ^
13:48 < zyga> mborzecki, in snap/validate.go:446 we should allow nix if the base is nix-base
13:49 < zyga> mborzecki, can you work with avn on getting this fixed
13:49 < zyga> nix-base was something that was brought up in Montreal a few years back
13:49 < zyga> but I guess it's not tested as a working base

1 List of permitted toplevel items -- https://github.com/snapcore/snapd/blob/master/snap/validate.go#L446
2 nix-base snap -- source https://github.com/NixOS/snapd-nix-base and package in repo https://snapcraft.io/nix-base
3 related ticket in nixpkgs -- https://github.com/NixOS/nixpkgs/issues/100618
(I'd put there link on this ticket as well)