workflows: Fix permissions for release-sources job (llvm#100750)

For reusable workflows, the called workflow cannot upgrade it's
permissions, and since the default permission is none, we need to
explicitly declare 'contents: read' when calling the release-sources
workflow.

Fixes the error:
The workflow is requesting 'contents: read', but is only allowed
'contents: none'.

(cherry picked from commit 82c2259)

Author: tstellar

.github/workflows/release-tasks.yml

@@ -101,6 +101,7 @@ jobs:
   release-sources:
     name: Package Release Sources
     permissions:
+      contents: read
       id-token: write
       attestations: write
     needs: