`nix-collect-garbage --dry-run` previously elided the entire garbage
collection check, meaning that it would just exit the script without
printing anything.
This change makes the dry run flag instead set the GC action to
`gcReturnDead` rather than `gcDeleteDead`, and then continue with the
script. So if you set `--dry-run`, it will print the paths it *would*
have garbage collected, but not actually delete them.
I filed a bug for this: lix-project/lix#432 but then realised I could give fixing it a go myself.
Change-Id: I062dbf1a80bbab192b5fd0b3a453a0b555ad16f2
