test: reject an unset pnpm home in smoke checks

.github/workflows/test.yaml

@@ -7,6 +7,9 @@ on:
       - master
   workflow_dispatch:
 
+permissions:
+  contents: read
+
 jobs:
   smoke:
     # Cross-OS coverage. Exercises the bootstrap install + PATH on each platform,
@@ -71,7 +74,7 @@ jobs:
           which pnpm
           which pnpx
           case "$PNPM_HOME" in
-            *node_modules*) echo "PNPM_HOME must be outside node_modules"; exit 1 ;;
+            ''|*node_modules*) echo "PNPM_HOME must be set outside node_modules"; exit 1 ;;
           esac
           actual="$(pnpm --version)"
           echo "pnpm --version: ${actual}"