Select the CSP nonce directive via startsWith("script-src") /
startsWith("default-src"), matching Next.js get-script-nonce-from-header.
Previously the exact-match predicate skipped script-src-elem/script-src-attr
directives, so a CSP declaring the nonce only on script-src-elem fell through
to default-src and SSR scripts could be emitted without the expected nonce
and blocked by the browser CSP.
Closes#1989