set explicit permissions for GitHub workflows - soundness.yml (swiftlang#168)

Signed-off-by: Melissa Kilby <mkilby@apple.com>

Author: incertum

.github/workflows/soundness.yml

@@ -80,6 +80,9 @@ on:
         description: "Linux command to execute before building the Swift package"
         default: ""
 
+permissions:
+  contents: read
+
 ## We are cancelling previously triggered workflow runs
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}-soundness