trace the packaged host runtime check

flake.nix

@@ -99,7 +99,8 @@
         checks.runtime-toolchain = pkgs.runCommand "sausage-runtime-toolchain" {
           nativeBuildInputs = [ pkgs.expect ];
         } ''
-          set -eu
+          set -eux
+          echo "runtime-check: prepare fixture"
           root="$TMPDIR/runtime-toolchain"
           world="$root/world"
           emptyPath="$root/empty-bin"
@@ -125,6 +126,7 @@
           export SAUSAGE_BIN=${sausage}/bin/sausage
           export WORLD="$world"
           export EMPTY_PATH="$emptyPath"
+          echo "runtime-check: start expect with empty PATH"
           expect <<'EOF'
           log_user 1
           set timeout 300
@@ -132,6 +134,7 @@
           set env(HOME) $::env(WORLD)
           set env(TMPDIR) [file dirname $::env(WORLD)]
           set env(CARGO_HOME) [file join [file dirname $::env(WORLD)] cargo-home]
+          puts stderr "runtime-check: spawn packaged host"
           spawn $::env(SAUSAGE_BIN) run $::env(WORLD)
           send -- "@cargo {\"type\":\"build\",\"source\":\"/actor/console/public/probe\"}\r"
           expect {