fix(ci): replace GITHUB_TOKEN by fine-grained one

.github/workflows/build.yml

@@ -114,7 +114,30 @@ jobs:
           CHISEL_VERSION: ${{ steps.build.outputs.CHISEL_VERSION }}
           ARCHIVE_FILE: ${{ steps.archive.outputs.ARCHIVE_FILE }}
           ARCHIVE_FILE_SHA384: ${{ steps.archive.outputs.ARCHIVE_FILE_SHA384 }}
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          # If triggered by a "release" event, this additional and final step
+          # of the job will upload the Chisel binaries (and checksums) to the corresponding
+          # GitHub release. This operation NEEDS "contents: write" permissions.
+          # Security concerns:
+          #  - Why a custom token instead of GITHUB_TOKEN?
+          #    - The GITHUB_TOKEN only has "readonly" permissions by default.
+          #  - Why not add "permissions: {contents:write}" to this workflow [1]?
+          #    - While this would elevate the GITHUB_TOKEN permissions to what we require, it
+          #      would do it for the entire job (ALL steps), which increases the attack surface unnecessarily.
+          #  - Why is it safer to use the ROCKSBOT_CHISEL_CONTENTS token?
+          #    1. it is limited to this step of the workflow (unlike GITHUB_TOKEN, which is always available)
+          #    2. this particular step only runs when the "event == release"
+          #    3. the step's script doesn't rely on 3rd party actions
+          #    4. the current workflow triggers are all maintainer-driven, except for "pull_request",
+          #       but the latter is designed not to use any secrets during execution from fork-based PRs
+          #        - NOTE: in fact, custom secrets are never passed to fork-based PR workflows, unlike GITHUB_TOKEN
+          #          which is available with "readonly" permissions [2]
+          # - How could this token be compromised?
+          #   - By unintentionally merging a change that allows this step to run unvetted scripts
+          #     (either via a "pull_request_target", use of an insecure 3rd party action, or malicious changes to the
+          #     current script)
+          # [1] https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-syntax#permissions
+          # [2] https://docs.github.com/en/actions/reference/workflows-and-actions/events-that-trigger-workflows#workflows-in-forked-repositories
+          GITHUB_TOKEN: ${{ secrets.ROCKSBOT_CHISEL_CONTENTS }}
         if: ${{ github.event_name == 'release' }}
         run: |
           echo "Uploading $ARCHIVE_FILE to release $CHISEL_VERSION"