This sets permissions: contents: read for the CI workflow. The matching change in Bitcoin Core is bitcoin/bitcoin#36513, for bitcoin/bitcoin#36512.
Part of #1959.
Give GITHUB_TOKEN only `contents: read`.
Part of #1959.
I dropped persist-credentials: false to stay in sync with Core, where review found it adds nothing once the token is read-only (https://github.com/bitcoin/bitcoin/pull/36513#issuecomment-6146223556). The PR now only sets permissions: contents: read.