Add option to create a 'result' symlink to colmena build

[f2k1de]

Using nix build results in a 'result' symlink to the built system configuration. This is useful if you want to compare that one with another using for example nix store diff-closures.
However colmena does not create such a symlink, it is required to manually parse the terminal output to get the location of the freshly built system.

[bjornfor]

If you run colmena apply --keep-result there will be symlinks to the top-level NixOS closures in $PWD/.gcroots/node-NAME.