bug#47115: Failure building grub-img.png when reconfiguring

[Jack Hill]

Still on the same VM, I've been able to reproduce the problem while 
building a different derivation: 
/gnu/store/07xw2pp63xin4c4y8ndrcdn3n8z1vmx2-grub-image.png.drv from Guix 
commit d4e29f3628ad0c7576d7cab659d7fcc19d21999a. I can still build the new 
derivation on my desktop.

Hrm, it's a pretty spooky problem.
Best,
Jack