ci: Set explicit content write permission for release workflow

This commit updates the release workflow such that an explicit content
write permission is granted to the GITHUB_TOKEN used throughout the
workflow.

The content write permission is required to upload the release assets
to the repository and must be manually set when the repository default
workflow permissions are set to "Read repository contents permission."

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio

.github/workflows/release.yml

@@ -6,6 +6,9 @@ on:
   release:
     types: [ published ]
 
+permissions:
+  contents: write
+
 env:
   BUNDLE_PREFIX: zephyr-sdk