ci: install further optional dependencies for demos

Achieved 100% pass rate for demo tests, at least on macOS.

.github/workflows/test_demo.sh

@@ -1,4 +1,5 @@
 #!/bin/bash
+set -e
 
 # llgo run subdirectories under _demo and _pydemo
 total=0
@@ -8,7 +9,7 @@ for d in ./_demo/* ./_pydemo/*; do
   total=$((total+1))
   if [ -d "$d" ]; then
     echo "Testing $d"
-    if ! llgo run "$d"; then
+    if ! (cd "$d" && llgo run .); then
       echo "FAIL"
       failed=$((failed+1))
       failed_cases="$failed_cases\n* :x: $d"