fix(tvix/nar-bridge): set FileSize to NarSize

Nix and Lix both report this quite wrong when showing progress, they
assume 0.00 MiB of download size:

```
these 3 derivations will be built:
  /nix/store/m6dkr67hk87lpcz9wv8f2mp2zrgsyfp9-nix-2.24.11.drv
  /nix/store/zcfi6vs0z18309asw8fpa9v3665av44v-niv-0.2.22.drv
  /nix/store/g4kvzrs1kk9i13vaa8q1r0y4hgsqsnlp-dev-env.drv
these 112 paths will be fetched (0.00 MiB download, 2649.76 MiB unpacked):
  /nix/store/3qzlg8h5qc1slypy99aafdcgkzj6974h-OneTuple-0.4.2
  /nix/store/pp4540rig52fnj66kz1kiaj1000ja9v0-QuickCheck-2.14.3
  /nix/store/416aqss6p59w6v92127hkz04v0bclx21-StateVar-1.2.2
  /nix/store/b5f93spm2cl9g6x73dnx7ns5irs739fz-aeson-2.1.2.1
  /nix/store/nqnx6k6y103rargdz2gai2rmi389zn6n-aeson-pretty-0.8.10
  /nix/store/11wc4s6a6qi98lxikacw746slhmj5dl7-ansi-terminal-1.0.2
  /nix/store/yy7j66av9lwh3lvbxp1zv7572wb4l7dj-ansi-terminal-types-0.11.5
  /nix/store/cn6m459cbacdwkvjllgy5hkzf045yc1g-appar-0.1.8
  […]
```

For now, set FileSize to NarSize - it's not more wrong than it was
before, Nix already supports content encoding for compression (via
curl).

Reported-On: https://git.lix.systems/lix-project/lix/issues/606
Change-Id: Ia53506ecf6678ad298f759c95a69feb441cbc26d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12919
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/nar-bridge/src/narinfo.rs

@@ -153,6 +153,10 @@ fn gen_narinfo_str(path_info: &PathInfo) -> String {
         path_info.nar_size,
     );
     narinfo.url = &url;
+
+    // Set FileSize to NarSize, as otherwise progress reporting in Nix looks very broken
+    narinfo.file_size = Some(narinfo.nar_size);
+
     narinfo.to_string()
 }