Permission denied on `*.chroot` when running as non-root user

[winterqt]

I unfortunately lost my log for this, but when running nix-heuristic-gc 0.7.1 as a non-root user, I got Permission denied on a /nix/store/*.chroot directory from a Python function (so before any of the actual store path deletions occur).

[risicle]

Innnnteresting.

I do catch PermissionError and handle it, but clearly not in enough places.

[winterqt]

Hopefully I can repro at some point, sorry that I lost the log :(

[risicle]

Just pushed a commit to master catching PermissionError in more cases, should be safe enough. Painful project to write tests for though.