feat(pkgs/risc0-rust): Output a toolchain directory, in the format rzup expects

With RISC0 commit 32c29a rzup also becomes a library, and extraction of
the rust binary path is done via it.

Author: Syndamia

packages/risc0-rust/default.nix

@@ -17,8 +17,13 @@ stdenv.mkDerivation rec {
 
   installPhase = ''
     runHook preInstall
-    mkdir -p $out
-    cp -r ./* $out/
+
+    mkdir -p "$out"
+    cp -r ./* "$out/"
+
+    mkdir -p "$out/r0.${version}-risc0-rust-x86_64-unknown-linux-gnu"
+    cp -r ./* "$out/r0.${version}-risc0-rust-x86_64-unknown-linux-gnu"
+
     runHook postInstall
   '';