Change how we download Keys?

[rootzoll]

see https://github.com/raspiblitz/raspiblitz/pull/3754/#issuecomment-1508119608

[openoms]

Joininbox has the improved method implemented already: https://github.com/openoms/joininbox/blob/a596e25ce03adfe48cf465fc01c4757167452ac1/scripts/_functions.bitcoincore.sh#L9-L92

Basically:

# Receive signer keys
curl -s "https://api.github.com/repos/bitcoin-core/guix.sigs/contents/builder-keys" |
    jq -r '.[].download_url' | while read url; do curl -s "$url" | gpg --import; done

# verify the manifest
gpg --verify SHA256SUMS.asc