Bootstrap busybox from binaries

.gitignore

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+result

busybox.nix

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+let
+  busybox = import ./fetchurl.nix {
+    url = "https://www.busybox.net/downloads/binaries/1.35.0-x86_64-linux-musl/busybox";
+    outputHash = "sha256-mVOoYPn3r9NYiZjQf5AF2p7hXy1Q4kN5RSscc/PGpiY=";
+    executable = true;
+  };
+  busybox_ash = import ./fetchurl.nix {
+    url = "https://www.busybox.net/downloads/binaries/1.35.0-x86_64-linux-musl/busybox_ASH";
+    outputHash = "sha256-cgXsXhZxOE62NM2uYD4tkw4IA6vsiwolJjhw7zTyiIQ=";
+    executable = true;
+  };
+  busybox_cp = import ./fetchurl.nix {
+    url = "https://www.busybox.net/downloads/binaries/1.35.0-x86_64-linux-musl/busybox_CP";
+    outputHash = "sha256-SohMSgXijkdN8Y7LE522obQ0QUAs8xBEQl+p0w8iAVc=";
+    executable = true;
+  };
+  busybox_mkdir = import ./fetchurl.nix {
+    url = "https://www.busybox.net/downloads/binaries/1.35.0-x86_64-linux-musl/busybox_MKDIR";
+    outputHash = "sha256-knwWrnrCqxv52cM78VW/00Zqju6YI0+kXo/ZPLzJb18=";
+    executable = true;
+  };
+in {
+  busybox = builtins.derivation {
+    system = "x86_64-linux";
+    name = "busybox";
+    builder = busybox_ash;
+    args = [ "-c" "${busybox_mkdir} $out; ${busybox_cp} ${busybox} $out/busybox" ];
+  };
+}

fetchurl.nix

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+{ url
+, outputHash
+, executable ? false
+, unpack ? false
+, name ? baseNameOf (toString url)
+}:
+
+derivation ({
+  builder = "builtin:fetchurl";
+  outputHashMode = "recursive";
+
+  inherit name url executable unpack outputHash;
+
+  outputHashAlgo = if outputHash == "" then "sha256" else "";
+
+  system = "builtin";
+
+  # No need to double the amount of network traffic
+  preferLocalBuild = true;
+
+})