Can't have relative paths in NIX_PATH in restricted mode

[shlevy]

Since cbbd21e

$ nix-instantiate --eval -E '<nixpkgs>' -I . --restrict-eval
error: not an absolute path: '.'

This occurs even if, as in this case, the path lookup you perform is not found in the relative path you added.

The cause is that realisePath calls checkSourcePath even if requireAbsolutePath = false, and in restricted mode checkSourcePath calls canonPath, which checks for an absolute path.

I suspect the best thing to do here is simply remove the requireAbsolutePath parameter altogether and not use realisePath in findFile, switching back to realiseContext.

[shlevy]

Ack, this is worse than I thought:

error: not an absolute path: 'channel:nixos-unstable'

Emphasizes that realiseContext is probably the right call. This value simply isn't necessarily a path.