fuuko/drone-runner-exec: Use unstable nix

This also adds /etc/static as read-only path to the sandbox, since
otherwise /etc/nix/nix.conf can’t be read.

machines/fuuko/services/drone/runner-exec.nix

@@ -10,7 +10,7 @@ let
     git-lfs
     gnutar
     gzip
-    nix
+    nixUnstable
   ];
 in
 {
@@ -44,6 +44,7 @@ in
         "/etc/passwd:/etc/passwd"
         "/etc/ssh/ssh_known_hosts:/etc/ssh/ssh_known_hosts"
         "/etc/ssl/certs/ca-certificates.crt:/etc/ssl/certs/ca-certificates.crt"
+        "/etc/static"
         "/nix"
       ];
       ExecStart = "${pkgs.unstable.drone-runner-exec}/bin/drone-runner-exec";