gnome.gdm: Simplify DESTDIR hack

Let’s point `DESTDIR` to a path under the build directory rather than under "$out".
This will prevent `installPhase` from creating `$out` or any other output directory,
allowing us to install the outputs just by moving them out of `$DESTDIR` directly into the store with `mv`.
Of course, that will also require moving the installing of `etc` after the outputs are installed.

Since `$out` does not currently contain `etc` subdirectory, we can also just move `$DESTDIR/etc` directly to `$out` rather than copying it.
And even if `$out/etc` existed, we could just merge it with `cp --recursive` instead of `rsync` so `rsync` is not actually necessary.

The code remains written defensively against files accidentally being misplaced while shuffling them around – parent directories of targets are used as `mv` destinations so that the move fails loudly if the directory already exists, rather than being moved inside as e.g. `$out/etc/etc`.

While at it let’s also improve practices a bit:
- Quote command arguments.
- Move `DESTDIR` definition into `env` block.
- Add vertical space and clearer comments.
- Handle non-standard Nix store paths.

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