update: set explicit permissions for GitHub workflows - benchmarks (#3389)

See #3386
CC @FranzBusch @ktoso @Lukasa

Signed-off-by: Melissa Kilby <mkilby@apple.com>
Co-authored-by: Rick Newton-Rogers <rnro@apple.com>

Author: incertum

.github/workflows/benchmarks.yml

@@ -73,6 +73,9 @@ on:
         description: "Environment variables for Linux jobs as JSON (e.g., '{\"DEBUG\":\"1\",\"LOG_LEVEL\":\"info\"}')."
         default: "{}"
 
+permissions:
+  contents: read
+
 jobs:
   construct-matrix:
     name: Construct Benchmarks matrix