Remove reliance on nixpkgs

lib/default.nix

@@ -1,45 +1,18 @@
 system: { nixpkgs, ... } @ args: rec {
-  pkgs = import nixpkgs { inherit system; };
   inherit (builtins) toFile;
 
-  runCommandBasic = script: builtins.derivation {
-    inherit system;
-    name = "runCommandBasic";
-    builder = "${pkgs.busybox}/bin/sh";
-    args = [ (toFile "builder.sh" script) ];
-  };
-
-  writeTextFile =
-    { name
-    , text
-    , executable ? false
-    , destination ? ""
-    , checkPhase ? ""
-    }: runCommandBasic ''
-      target=$out${destination}
-      mkdir -p "$(dirname "$target")"
-
-      if [ -e "$textPath" ]; then
-        mv "$textPath" "$target"
-      else
-        echo -n "$text" > "$target"
-      fi
-
-      eval "$checkPhase"
-      (test -n "$executable" && chmod +x "$target") || true
-    '';
-  writeText = name: text: writeTextFile { inherit name text; };
-  writeTextDir = path: text: writeTextFile {
-    inherit text;
-    name = builtins.baseNameOf path;
-    destination = "/${path}";
-  };
-  writeScript = name: text: writeTextFile { inherit name text; executable = true; };
-  writeScriptBin = name: text: writeTextFile { inherit name text; executable = true; destination = "/bin/${name}"; };
-
   baseDerivation = { script, ... } @args: builtins.derivation (args // {
     inherit system;
-    builder = "${pkgs.busybox}/bin/sh";
+    builder = ./. + "/../prebuilt/${system}/busybox";
-    args = [ (writeScript "${args.name}-builder.sh" script) ];
+    args = [
+      "sh"
+      (toFile "${args.name}-builder.sh" (
+        ''
+          set -ex
+          export PATH=${./. + "/../prebuilt/${system}"}
+          eval "$script"
+        ''
+      ))
+    ];
   });
 }

prebuilt/armv7l-linux/cp

@@ -0,0 +1 @@
+busybox

prebuilt/armv7l-linux/dirname

@@ -0,0 +1 @@
+busybox

prebuilt/armv7l-linux/echo

@@ -0,0 +1 @@
+busybox

prebuilt/armv7l-linux/mkdir

@@ -0,0 +1 @@
+busybox

prebuilt/armv7l-linux/mv

@@ -0,0 +1 @@
+busybox