nixos-container destroy: Remove gcroots

(cherry picked from commit 883fa4f9205b2aa699760a73e1f1a26bb4f5296e)
nixos-container destroy: Make idempotent

(cherry picked from commit e6c00e60c3eb6ba390e2081818d7ac30c916d3e0)
Revert "Turn some license strings into lib.licenses values"

This reverts commit f9f5be6113105626cea102763efe871ce521dd50 because
it breaks Nixpkgs evaluation.

http://hydra.nixos.org/build/13476850
Re-revert the previous and fix eval

This reverts commit 155996ebde2a9042b0252954245d298008755dc5.