guix: use --build={arch}-guix-linux-gnu in cross toolchain
guix: use --build={arch}-guix-linux-gnu in cross toolchain

Technically we are always cross-compiling, so make that explicit.

Fixes: #22458.

sstone/bitcoin,bitcoinsSG/bitcoin,tecnovert/particl-core,fanquake/bitcoin,sstone/bitcoin,ajtowns/bitcoin,tecnovert/particl-core,lateminer/bitcoin,tecnovert/particl-core,bitcoin/bitcoin,mruddy/bitcoin,jamesob/bitcoin,jambolo/bitcoin,AkioNak/bitcoin,particl/particl-core,AkioNak/bitcoin,fujicoin/fujicoin,bitcoinsSG/bitcoin,jamesob/bitcoin,bitcoin/bitcoin,particl/particl-core,sstone/bitcoin,fujicoin/fujicoin,namecoin/namecoin-core,kallewoof/bitcoin,jamesob/bitcoin,fanquake/bitcoin,kallewoof/bitcoin,namecoin/namecore,AkioNak/bitcoin,fanquake/bitcoin,AkioNak/bitcoin,namecoin/namecoin-core,tecnovert/particl-core,bitcoinsSG/bitcoin,fujicoin/fujicoin,lateminer/bitcoin,bitcoinsSG/bitcoin,sstone/bitcoin,jambolo/bitcoin,ajtowns/bitcoin,kallewoof/bitcoin,fanquake/bitcoin,lateminer/bitcoin,namecoin/namecore,jambolo/bitcoin,lateminer/bitcoin,mruddy/bitcoin,sipsorcery/bitcoin,kallewoof/bitcoin,ajtowns/bitcoin,bitcoinsSG/bitcoin,bitcoin/bitcoin,tecnovert/particl-core,particl/particl-core,mruddy/bitcoin,ajtowns/bitcoin,sipsorcery/bitcoin,sipsorcery/bitcoin,bitcoin/bitcoin,namecoin/namecore,lateminer/bitcoin,AkioNak/bitcoin,sstone/bitcoin,jambolo/bitcoin,fanquake/bitcoin,bitcoin/bitcoin,particl/particl-core,jamesob/bitcoin,bitcoin/bitcoin,fujicoin/fujicoin,kallewoof/bitcoin,namecoin/namecore,namecoin/namecoin-core,sipsorcery/bitcoin,namecoin/namecore,AkioNak/bitcoin,namecoin/namecoin-core,lateminer/bitcoin,fujicoin/fujicoin,particl/particl-core,ajtowns/bitcoin,jambolo/bitcoin,fanquake/bitcoin,kallewoof/bitcoin,mruddy/bitcoin,particl/particl-core,bitcoinsSG/bitcoin,jamesob/bitcoin,namecoin/namecore,namecoin/namecoin-core,sstone/bitcoin,ajtowns/bitcoin,jamesob/bitcoin,namecoin/namecoin-core,fujicoin/fujicoin,sipsorcery/bitcoin,mruddy/bitcoin,mruddy/bitcoin,sipsorcery/bitcoin,tecnovert/particl-core,jambolo/bitcoin