refactor(3p/nix): Build nix-daemon as a separate binary

Skips over all the monobinary stuff and moves to a separate binary for
nix-daemon.

This also replaces the flag parsing logic with absl::flags. This
causes a behaviour change for --help, which no longer tries to display
a man page but instead shows the actual command-line help.

Note: This binary no longer links to the Boehm GC.

Change-Id: Ib852e994b82f2d56e91262878c10650e656427a9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1622
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>

third_party/nix/default.nix

@@ -121,7 +121,6 @@ in lib.fix (self: pkgs.llvmPackages.libcxxStdenv.mkDerivation {
     ln -s $out/bin/nix $out/bin/nix-channel
     ln -s $out/bin/nix $out/bin/nix-collect-garbage
     ln -s $out/bin/nix $out/bin/nix-copy-closure
-    ln -s $out/bin/nix $out/bin/nix-daemon
     ln -s $out/bin/nix $out/bin/nix-env
     ln -s $out/bin/nix $out/bin/nix-hash
     ln -s $out/bin/nix $out/bin/nix-instantiate