bug#65225: ‘guix shell --system=ALIEN’ builds for both systems

[Tobias Geerinckx-Rice]

Hi,

ekaitz noticed this when building a mes-derived package that fails on 
x86_64.  Here's a reproducer that uses upstream Guix:

$ guix shell --system=riscv64-linux drawterm --no-grafts --rebuild-cache 
--dry-run | grep drv$
  /gnu/store/4c02fgswkbldys93w4vgj0gwax2ly4bh-profile.drv

$ guix build --dry-run 
/gnu/store/4c02fgswkbldys93w4vgj0gwax2ly4bh-profile.drv | grep drawterm
  
/gnu/store/mkxyhjq117wdalc0gbz4468blqxih1kn-drawterm-20210628-1.c97fe46
  
/gnu/store/h8dk35aw2n8rj1hki6dkdrknfly26vq8-drawterm-20210628-1.c97fe46

/gnu/store/fl4iimlcdnlkarjm8m6z3392wss6b8yr-drawterm-20210628-1.c97fe46.drv
→ 
/gnu/store/mkxyhjq117wdalc0gbz4468blqxih1kn-drawterm-20210628-1.c97fe46
is riscv64-linux.

/gnu/store/9a1pji59hzacrmpb65nk3pp3m01niqf3-drawterm-20210628-1.c97fe46.drv
→ 
/gnu/store/h8dk35aw2n8rj1hki6dkdrknfly26vq8-drawterm-20210628-1.c97fe46
is x86_64-linux.

Eventually the ‘correct’ riscv64 drawterm ends up in the shell, but Guix 
should never have built the x86_64 version.

Kind regards,

T G-R

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