Preliminary NixOS rebuilding support

Taking pkgs specs of the form:

```
[
  {
    "name": "zlib",
    "version": "1.3@q9jf8xl1gsfmx4f1jhqpdc11yf0w9gv4",
    "distro": "nixos",
    "suite": "core",
    "architecture": "x86_64",
    "input_url": "https://cache.nixos.org/q9jf8xl1gsfmx4f1jhqpdc11yf0w9gv4.narinfo",
    "artifacts": [
      {
        "name": "zlib",
        "version": "1.3",
        "url": "https://cache.nixos.org/nar/1dv9dwz64z347s8b5s78lqrbgdl4xcm0l98zd9cm7jnz1ljd1gjy.nar.xz"
      }
    ]
  }
]
```

Yay:

```
Comparing "/tmp/rebuilderd03zPu6/inputs/1dv9dwz64z347s8b5s78lqrbgdl4xcm0l98zd9cm7jnz1ljd1gjy.nar.xz" with "/tmp/rebuilderd03zPu6/out/1dv9dwz64z347s8b5s78lqrbgdl4xcm0l98zd9cm7jnz1ljd1gjy.nar.xz"
```

contrib/confs/rebuilderd-worker.conf

@@ -31,3 +31,6 @@ path = "/usr/libexec/rebuilderd/rebuilder-debian.sh"
 
 [backend."tails"]
 path = "/usr/libexec/rebuilderd/rebuilder-tails.sh"
+
+[backend."nixos"]
+path = "/usr/libexec/rebuilderd/rebuilder-nixos.sh"

docker-compose.yml

@@ -25,6 +25,18 @@ services:
     - REBUILDERD_COOKIE_PATH=/secret/auth
     volumes:
     - ./secret:/secret
+  worker-nixos:
+    build:
+      context: .
+      dockerfile: worker/Dockerfile.nixos
+    # number of concurrent workers
+    scale: 1
+    init: true
+    command: ['connect', 'http://daemon:8484']
+    environment:
+    - REBUILDERD_COOKIE_PATH=/secret/auth
+    volumes:
+    - ./secret:/secret
   worker-archlinux:
     build:
       context: .

worker/rebuilder-nixos.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+
+set -x
+
+echo "Rebuilding $1"
+export PATH=/root/.nix-profile/bin:$PATH
+whoami
+
+DRV=$(cat $1 | grep StorePath | cut -d ":" -f 2)
+
+REALIZED=$(nix-build --check $DRV)
+
+nix-store --dump $REALIZED > $REBUILDERD_OUTDIR/out.nar
+xz $REBUILDERD_OUTDIR/out.nar
+HASH=$(nix-hash --base32 --type sha256 --flat $REBUILDERD_OUTDIR/out.nar.xz)
+mv $REBUILDERD_OUTDIR/out.nar.xz $REBUILDERD_OUTDIR/$HASH.nar.xz