Fixes two bugs:
- pass a forgotten `{}` to `compressDrv`.
- remove incorrect usage of `lndir` in `compressDrv`. I added a brief
comment on why, see [this comment][1] for more details.
Tested with:
```
$ nix build .#legacyPackages.x86_64-linux.gitea.passthru.data-compressed
$ ls -lh result/public/assets/licenses.txt*
lrwxrwxrwx 1 root root 90 Jan 1 1970 result/public/assets/licenses.txt -> /nix/store/p21irsr57hckd3x3ym18aa0cr9zmm3an-gitea-1.22.1-data/./public/assets/licenses.txt
-r--r--r-- 1 root root 30K Jan 1 1970 result/public/assets/licenses.txt.br
-r--r--r-- 1 root root 82K Jan 1 1970 result/public/assets/licenses.txt.gz
```
[1]: https://github.com/NixOS/nixpkgs/pull/332752#discussion_r1721043286
*compressDrv* compresses files in a given derivation.
*compressDrvWeb* compresses a derivation for a loosely-defined
pre-compressed "web server" usage.
This intends to replace the `passthru.data-compressed` derivations that
have accumulated in nixpkgs with something more reusable.
