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

[Jack Hill]

Hi Guix,

When reconfiguring my system, the build of 
/gnu/store/0yf1b1l19h7c3jj1zkhxjmq4sb3yysjq-grub-image.png.drv failed with 
the following:

```
Backtrace:
           2 (primitive-load "/gnu/store/larqpc2wjhnc6jmj4885k8lynd1?")
In gnu/build/svg.scm:
     53:6  1 (svg->png _ "/gnu/store/xadbzis4pvmxib4fk55jrag4fmn55w?" ?)
In unknown file:
           0 (rsvg-handle-render-cairo #<rsvg-handle 7ffff5b60150> #)

ERROR: In procedure rsvg-handle-render-cairo:
Wrong type (expecting finalized smob): #<cairo-context 7ffff5b60090>
```

This is with Guix bb5d84a0489a629d30bc2e978807caf20f46e329. My last 
successful reconfigure was with 80739ea480a7db667b83b45e3a08be740449f689. 
The output of the reconfigure run is attached. Reconfiguring without 
grafts succeeds.

Best,
Jack

reconfigure-log
Description: Text document

no-grafts
Description: Text document

config.scm
Description: Text document