fix(pkgs/jolt): Fix cargo script RUSTUP_TOOLCHAIN logic

When the guest is compiled with the standard library, cargo is spawned
with RUSTUP_TOOLCHAIN variable set. In such cases we put the jolt rust
fork as the "default" cargo to call.
Before this commit, the condition worked in the opposite way.

Author: Syndamia

packages/jolt/default.nix

@@ -56,7 +56,7 @@ in
         rm $out/bin/cargo
         cat <<EOF > $out/bin/cargo
         #!/bin/sh
-        if [ -z "\''${RUSTUP_TOOLCHAIN+x}" ]
+        if [ -n "\''${RUSTUP_TOOLCHAIN+x}" ]
         then
             export PATH="${jolt-guest-rust}/rust/build/host/stage2/bin:$PATH"
         fi