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$ nix-build -A c
these derivations will be built:
  /nix/store/rnwk48gkwx5gyd9f1rs9km90va2dyxfm-c.drv
building path(s) ‘/nix/store/zjhl38k054fxq3psqcnbncmqyry1310w-c’
cannot open `/nix/store/sn7v6cp5gif0pakxj00w6260nvvd2iky-a/dir1/systemd-private-c82314a459a84288881152c90884ba80-tor.service-5E7iLv': Permission denied at /nix/store/9g4wc31j7a2xp22xpgwr0qssfxahxdzl-builder.pl line 58.
builder for ‘/nix/store/rnwk48gkwx5gyd9f1rs9km90va2dyxfm-c.drv’ failed with exit code 13
error: build of ‘/nix/store/rnwk48gkwx5gyd9f1rs9km90va2dyxfm-c.drv’ failed

with import <nixpkgs> {};

rec {
    a = runCommand "a" {} ''
      mkdir -p $out
      ln -sv /var/tmp $out/dir1
    '';
    b = runCommand "b" {} ''
      mkdir -p $out
      ln -sv /var/tmp $out/dir1
    '';
    c = buildEnv {
      name = "c";
      paths = [ a b ];
    };
}