build-support/rust: Split out sysroot src derivation

Hoping to make it usable for `buildRustCrate` too.

pkgs/build-support/rust/build-rust-package/sysroot/default.nix

@@ -3,17 +3,8 @@
 { shortTarget, originalCargoToml, target, RUSTFLAGS }:
 
 let
-  cargoSrc = stdenv.mkDerivation {
-    name = "cargo-src";
-    preferLocalBuild = true;
-    phases = [ "installPhase" ];
-    installPhase = ''
-      RUSTC_SRC=${rustPlatform.rustcSrc.override { minimalContent = false; }} ORIG_CARGO=${originalCargoToml} \
-        ${buildPackages.python3.withPackages (ps: with ps; [ toml ])}/bin/python3 ${./cargo.py}
-      mkdir -p $out
-      cp Cargo.toml $out/Cargo.toml
-      cp ${./Cargo.lock} $out/Cargo.lock
-    '';
+  cargoSrc = import ../../sysroot/src.nix {
+    inherit stdenv rustPlatform buildPackages originalCargoToml;
   };
 in rustPlatform.buildRustPackage {
   inherit target RUSTFLAGS;

pkgs/build-support/rust/sysroot/src.nix

@@ -0,0 +1,21 @@
+{ lib, stdenv, rustPlatform, buildPackages
+, originalCargoToml ? null
+}:
+
+stdenv.mkDerivation {
+  name = "cargo-src";
+  preferLocalBuild = true;
+  phases = [ "installPhase" ];
+  installPhase = ''
+    export RUSTC_SRC=${rustPlatform.rustcSrc.override { minimalContent = false; }}
+  ''
+  + lib.optionalString (originalCargoToml != null) ''
+    export ORIG_CARGO=${originalCargoToml}
+  ''
+  + ''
+    ${buildPackages.python3.withPackages (ps: with ps; [ toml ])}/bin/python3 ${./cargo.py}
+    mkdir -p $out
+    cp Cargo.toml $out/Cargo.toml
+    cp ${./Cargo.lock} $out/Cargo.lock
+  '';
+}