Grant build-compiler the permission to delete caches

Codex review: the job relied on the repository's default token
permissions for the cache deletions of master pushes, which now fail
instead of being ignored.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Kbf3gKHFTuMrZosy3Nb5Qc
Signed-off-by: Christoph Knittel <ck@cca.io>

Author: cknitt

.github/workflows/ci.yml

@@ -28,6 +28,9 @@ env:
 
 jobs:
   build-compiler:
+    permissions:
+      contents: read
+      actions: write # Master pushes delete superseded caches.
     outputs:
       api-docs-artifact-id: ${{ steps.upload-api-docs.outputs.artifact-id }}
       website-branch: ${{ steps.api-docs-plan.outputs.website-branch }}