update: set explicit permissions for GitHub workflows - cxx-interop (#3387)

See #3386
CC @FranzBusch @ktoso @Lukasa

Signed-off-by: Melissa Kilby <[email protected]>

Author: incertum

.github/workflows/cxx_interop.yml

@@ -70,6 +70,9 @@ on:
         description: "Environment variables for Windows jobs as JSON (e.g., '{\"DEBUG\":\"1\",\"LOG_LEVEL\":\"info\"}')."
         default: "{}"
 
+permissions:
+  contents: read
+
 jobs:
   construct-matrix:
     name: Construct Cxx interop matrix